Croatia

Reasons to go include:

1. Northern Velebit National Park

★ Premužić trail is perhaps the best hike in Croatia. From the parking at (44.813949, 14.978578) it is a nice forest walk to the beginning of a hike at (44.803438, 14.982182). The most dramatic landscapes are between (44.787431, 14.986748) and (44.760471, 14.989368), at which point I turned back. Continuing along the hike to climb Krajačev kuk (44.750973, 14.995473) is on my wishlist.
You could buy the national park ticket (cash only) at Babić Siča entrance (44.839721, 14.966143), but it is much more convenient to buy it on the national park’s website and not worry about the opening times of the ticket office.

2. Zagreb

Architecture along the Green Horseshoe: the buildings at this intersection (45.812273, 15.981665), this one (45.81228, 15.977717), this one (45.810596, 15.976445), buildings along Đorđićeva street including (45.810653, 15.979176), (45.810791, 15.980457) and wonderfully pretty (45.810627, 15.982873), this one (45.807752, 15.980322), Art Pavilion in Zagreb, Zagreb Glavni Kolod Train Station, this one (45.805896, 15.975904), Croatian State Archives, Ethnography Museum, Croatian National Theatre. Also don’t miss the Meteorological Post, wavy Esplanade Fountain (45.805288, 15.977262) and The Well Of Life fountain (45.810037, 15.970008). Park Kralja Petra Svačića is nice.
Cvjetni trg (45.812222, 15.974096) - perhaps my favourite square in Zagreb.
The model of the Solar System to scale, starting from the sun at (45.812187, 15.975469). While looking at the sun, don’t miss the beautiful buildings surrounding it and the Oktogon stained glass roof nearby.
Old Town: St. Mark’s Church (see it from both sides), Cathedral of Zagreb, Stone Gate (Virgin Mary shrine). In the evening, Ul. Ivana Tkalčića is the party street.
Tunel Grič - connecting (45.81478, 15.975611) and (45.81491, 15.971014), free. It also contains the most atmospheric toilet (45.814194, 15.972344) in Zagreb. And don’t miss this viewpoint (45.814346, 15.972012), and this one (45.814202, 15.975048).
On my wishlist: Mirogoj cemetery, Park Maksimir and all the art museums (Art Pavilion in Zagreb, Mimara Museum, National Museum of Modern Art, Klovićevi Dvori Gallery, Croatian Association of Fine Artists, Museum of Contemporary Art, Croatian Museum of Naïve Art).

Zagreb notes:

The area around Croatian Association of Fine Artists and Martićeva street is the best place to stay. Nearby Konzum (45.812244, 15.983754) stocks everything I wanted.
Coffee: Luta Specialty Coffee Shop (I heard it might be changing name and/or location soon) and Broom44 are my favourites. Four Wheel Coffee Roasters (order on the second floor to drink on the fourth floor terrace) and Cogito Coffee (several locations) are local roasteries, their coffee is ok. Other places that might be worth a try: Holographik Space, Monocycle specialty coffee, Eli’s caffe, Filteraj specialty coffee.
Food places that might be worth a try: Broom44 and Salo early in the day, Pod Zidom Bistro for dinner.
Grocery shopping: Bread Club (several locations) for sourdough, Tržnica Kvatrić (market without tourists) for fresh fruit and vegetables, big Konzum (45.805039, 15.977593) underneath the Kolodvorski Park (walk downstairs) for things you can’t find in smaller supermarkets.
Autonomni kulturni centar Medika (45.806336, 15.9644) is a cool party venue.
Zagreb is still recovering from the 2020 earthquake.

3. Split

Diocletian’s Palace is the most impressive part of the Old Town. It is part of the city - there are no opening hours and no tickets to enter. For maximum wow factor enter through the Bronze Gate (43.50753, 16.439765); early in the morning is best, before the crowds arrive. Don’t miss the impressive round Vestibul (43.507925, 16.43995).
Fun to explore squares: Narodni trg, Radić Brothers Square, Republic Square, Trg Šperun (43.508469, 16.434679).
Churches big and small: Saint Domnius Cathedral, Orthodox Church of St. Sava - entrance through (43.509386, 16.437628), Church of St. Nicholas (43.509216, 16.434348).
Modern side of Split: interesting building at Marmontova ulica 4 (43.509039, 16.436999), Pirja Fountain (43.510039, 16.437539), Park Josipa Jurja Strossmayera.
Marjan viewpoint (43.507705, 16.430329) - best viewpoint of the city.
Coffee: Stow Coffee Roasters Split and kava².
Food: Kruv Artisan Bakery, aROMA gelato (several locations), and Green Market for fruits and vegetables. For restaurants, I’d try Soul Kitchen.
We parked at (43.512553, 16.438028).

4. Plitvice Lakes National Park

This is the most popular national park in Croatia, which means that you should buy tickets well in advance and arrive at the opening time to avoid elbowing your way along the narrow board walks. The ticket includes admission through one of the two entrances (you have to select which one), unlimited short boat rides, one long boat ride and one bus ride inside the national park, but not the parking fees.
I wanted to start with the best viewpoint (44.904354, 15.610911) before it gets too crowded, see all the waterfalls and walk south facing the waterwalls, not the other way. Hence I bought a ticket for entrance 1 and mostly followed the signs for program C.
Entrance 1 parking is at (44.90625, 15.612546), toilets and ticket check at (44.905032, 15.611327). From there I tried to walk all the board walks. Don’t miss the Great Waterfall viewpoint (44.902393, 15.608752). To avoid long queues, try to get on the first boat from P3 (44.893467, 15.603707) to P2 (44.880087, 15.615407), check the schedule here. From P2 I continued walking all the board walks, which end at St3 (44.87091, 15.599343) where there are toilets and the bus to St1 (44.896648, 15.611909). On the walk from St1 back to the entrance 1 parking lot there is a great viewpoint at (44.899232, 15.611673).
Toilets: Apart from entrances 1 and 2, inside the national park there are toilets at boat station P3 and at bus station St3. There is no toilet at boat station P2, so you’ll need to cross to P1 (44.880387, 15.617193) for that. These boats are frequent and usually have no queues, so you can hop from P2 to P1 and back in about 15 minutes.
Expect that there might be long queues for boats between P3 and P2 and for buses; sometimes it is faster to walk it instead.
Some information about the waterfalls.

5. Rijeka

Architecture: apart from the Theatre Park with the view of Palace Modello and other great buildings, my favourite ones were Ribarnica Rijeka (seafood market with wonderful shrimps carved into the entrance columns), Italian Secondary School (45.328729, 14.43958), City Clock Tower, and Saint Vitus Cathedral (with a view of the prison).
★ Islamic Center of Rijeka (45.34587, 14.39744) - this mosque is perhaps my favourite example of modern Islamic architecture.
Rijeka Train Station - walk through to Platform 2 for the great view of the huge grain silos.
Torpedo Launch Station ruins. Park just outside the entrance (45.335523, 14.400994) and walk to (45.334318, 14.403386). It is totally ok to visit; local runners run this route, and the security guard will not ask you any questions.
On my wishlist: walking out onto the Mololongo (45.327092, 14.419096) and Trsat Castle (with the chicken-dragon statue).
Notes: EmJoy specialty coffee shop is good; Marsecchia Coffee Shop is on my wishlist. El Rio is a good Italian restaurant. We parked at (45.327968, 14.44271).

6. Other old towns, apart from Split

★ Trogir. The old town on the island is not big, but every street is very cute. St. Lawrence’s Cathedral is great. We parked at (43.517563, 16.247975) and crossed onto the old town island via the lovely Pješački most bridge.
Šibenik - a lovely old town without too many tourists. Cathedral of St. James is great, also see it from this high viewpoint (43.735935, 15.889413). Poljana square (43.734545, 15.893431) is a new addition to Šibenik, making it even nicer. Bava bistro is good (they have a couple of vegetarian options), Peperoncino Kitchen & bar also looked popular with locals. We parked at (43.73232, 15.894548).
Rovinj. Starting from the clock tower (45.082001, 13.634724), through Balbi’s Arch, up the Ul. Grisia street to St. Euphemia’s Church and the views from there. Church of the Most Holy Trinity is cute. Augusto Coffee Shop has the best coffee in the area.
Pula. Pula Arena is the main reason to visit Pula. Walking around this Roman amphitheater was enough for me, I didn’t pay for the ticket to enter. Once you are in Pula, don’t miss Mosaic The Punishment of Dirce (44.868762, 13.843923), Temple of Augustus (outside only), Bank of Italy Building (44.870195, 13.848291), Arch of the Sergii, Roman Mausoleum and view of the Uljanik shipyard. I skipped Zerostrasse tunnels.
Labin. A small old town on a hill. Enter through the City Gate St. Flora (45.085752, 14.12313), explore the narrow streets, exit through the Narrow Gate (45.085578, 14.124121) for the viewpoint.
Zadar. For me, Sea Organ is the only reason to visit Zadar. If you do end up in Zadar, don’t miss The Greeting to the Sun monument, Church of St Donatus and the Five Wells Square. We parked at (44.11077, 15.232106), entered the old town through the Land Gate, and sneaked into this free toilet (44.113603, 15.227776).

7. Abandoned architecture

★ Monument to the uprising of the people of Kordun and Banija (45.31623, 15.80499) - a huge abandoned building popular with locals on a weekend. Really fun to explore, amazing interiors in a dramatic state of decay. The satellite dishes on the roof are operational, so the staircase is used by engineers and hence maintained in a safe condition.
Dvigrad ruins - free to visit any time. Parking (45.126379, 13.813022), the main building (45.127096, 13.811986), a good view (45.127136, 13.812382), an old pizza oven (45.126649, 13.812005).
Frankopan Dvorac (45.482412, 15.452913) - a small castle. Free to visit, but there is a gate that could be closed.
Žakalj Old Mill ruins near Rijeka - park at (45.334886, 14.459461), cross the second bridge (45.337936, 14.464191) and enter the main building (45.338868, 14.46343).

Notes:

National Park tickets can be purchased on this official website.
Kent Bank ATMs do not charge for withdrawals.
Parking is not cheap; and reasonably priced parking lots are often full.
If you see a sign for a public toilet - it won’t be free.
Tap water is drinkable in Croatia. Public drinking water taps are common in urban areas.
Grocery shopping: By far the best place to buy fruits and vegetables is from a local market; most towns have at least one. For everything else, Konzum supermarkets are good. Nutty bars are great; I found them at Konzum.
I found Kami’s blog useful.