Thailand

1.1 King Power Mahanakhon building★

Outdoor observation deck on 78th floor is great (handbags only; tiny backpacks and water bottles not allowed; free lockers available). I arrived 2 hours before the sunset and stayed for 3 hours - the views just never get old. The glass floor balcony is awesome - you can lie on your stomach and look straight down on the city (no phones allowed on the balcony; you can go on it several times, just keep your shoe coverings).

The museum alone (on the second floor of the Simaseng Building) makes this place worth visiting.
Numerous snakes on the ground floor and outside are the main reason to visit.
You must visit on a workday to see the venom extraction demonstration (at 11am) and snake handling show (at 2pm) followed by an opportunity to handle a tame snake while they take a picture of you.
You can go out for lunch and come back with the same ticket. Arabica Bangkok Park Silom for coffee and Tonklarfacai (Chulalongkorn hospital branch) for lunch are good options. Note that Tonklarfacai (Aree branch) has a very different menu - mostly international dishes, not many Thai flavours.

1.3 Phra Nakhon district:

I only had time for one temple and I chose Wat Sa Ket Ratchaworamahawihan (The Golden Mount) - great views of the city and surrounding temples, opportunities to play the bells. Nearby Onibus coffee is good.
On my wishlist: The Grand Palace including Emerald Buddha, Pak Khlong Talat (Flower Market), Amulet Market, Wat Phra Chetuphon Wimon Mangkhalaram Rajwaramahawihan (temple), Wat Arun Ratchawararam Ratchawaramahawihan (temple on the other side of the river), May Kaidee Tanao Vegetarian & Vegan Restaurant and Ethos Vegetarian Restaurant.

1.4 Samphanthawong and Bang Rak districts:

Warehouse 30 - a restored warehouse with art galleries. ATTA Gallery and CURU Gallery were my favourite. Loads of sculptures outside Asia Collection Co.; and an artwork by VHILS opposite it. Soi Charoen Krung 32 - a street lined with street art.
Talat Noi Street Art area around (13.732909, 100.512907) and (13.733522, 100.512645). The nearby stretch of Soi Wanit 2 street is lined with shops selling car parts - fascinating piles of car bits! It culminates in the statue of a dragon (13.731917, 100.513634).
Ruyi vegetarian restaurant - great Thai flavours with imitation meat and seafood. Krua Boon Thai is also good. On my wishlist: Jae Lee Vegan (restaurant), So Vegan (restaurant), 511Cafe & restaurant Thai, CHATA Specialty Coffee (cafe).
Mahaniyom Cocktail Bar - great cocktails based on local flavours. Asia Today Bar (honey-centric cocktails) and a buzzing neighbourhood around it.
The skytrain ride from National Stadium station to Saphan Taksin station is architecturally exciting. Among other things, try to catch a glimpse of King Power Mahanakhon building. Once you arrive, other architectural gems await - State Tower Bangkok, General Post Office, etc.
Bangkok Art and Culture Centre (free) - a space with exhibitions and small galleries.
Wat Traimit Withayaram Worawihan (temple) looks impressive from the outside.
Sri Maha Mariamman Temple - Hindu temple, a must see if you’ve not yet been to India.

1.5 Watthana and Khlong Toei districts:

Na Aroon★ - an amazing restaurant (vegetarian and seafood). I usually avoid restaurants located in hotels, but this one justifies an exception.
theCOMMONS - an interesting take on a food court with a co-working feel. Seven Suns matcha cafe is a great place to try hojicha (the other type of powdered green tea).
Palette Artspace - several floors of gallery space on top of a cafe. Lost in Thaislation (cocktails based on food flavours) is on my wishlist.
Food: Khun Churn (vegetarian restaurant) and KIF (coffee) are great. May Veggie Home (vegan restaurant) and W Market (evening outdoor food court) are on my wishlist.
Benchakitti Forest Park - a huge park with wetlands, interesting plants and birds. I found nearby Sukhumvit Soi 4 deeply depressing, but easily avoidable.
One Bangkok was still under construction when I visited, I’m excited to see what it will develop into.

1.6 Phaya Thai district:

Area around Ari station is great place to stay.
My favourites were Aribica Specialty Coffee for coffee, Bamboo Garden Healthy Vegetarian Foods for lunch, Villa Market for grocery shopping. Plot Twist Bar (cocktails) is on my wishlist.

Notes:

Best time to visit Bangkok is November to February - after the rainy season and before it gets too hot.
Public transport is fragmented. Rechargeable Rabbit Card (one per person, passport required to purchase) can be used only on BTS Skytrain lines, but not on the subway system (MRT) or City Line Airport Rail Link. Subway system (MRT) works with bank card-based contactless payments.
Both Bolt and Grab taxi apps work in Bangkok. I had more success with Bolt, using cash as a payment method (drivers prefer it).
Villa Market is a great supermarket chain.
Visiting most temples requires both following a dress code (for women) and a purchase of a ticket.
In Suvarnabhumi Airport (BKK) I found edible food in Aim Deli (in the direction of F gates). I found a water refill station too.

The route I took (and loved):

Wat Yai Chai Mongkhon - observe locals dress the Buddha statues. Tickets at (14.346246, 100.59294).
Wat Maha That - an area with loads of Buddha body parts scattered around. Tickets at (14.356094, 100.56773).
Wat Ratcha Burana - the only place I found where you can go inside the ruins of the temple (and there are bats inside!). Tickets at (14.358779, 100.5684).
Wat Phra Si Sanphet - three big stupas in a line. Tickets at (14.355313, 100.55864). Don’t miss the giant Buddha statue in Wihan Phra Mongkhon Bophit nextdoors (14.354863, 100.55771; free).
Wat Lokkayasutha (free) and giant Reclining Buddha Sai Yat (14.355458, 100.55234; free).
Wat Chai Watthanaram - the place to be for sunset! Tickets at (14.343692, 100.54142). Many people use nearby costume rental services to dress up and make photos at sunset, hence the place is really atmospheric. I read about a viewpoint across the river from here, but I didn’t see anyone there, I don’t think there is access.

Notes:

None of the above can be seen on an elephant ride. The route for elephant rides goes around Wat Phra Ram, and elephants don’t look happy. Wat Phra Ram is visible from outside, so I didn’t buy the ticket to go inside the area.
Historic City of Ayutthaya makes for a great daytrip, even if you are on a very tight budget. You can get a very cheap train ticket from Krung Thep Aphiwat Central Terminal Station to Ayutthaya, then walk around or hire a bicycle. Most ruins are visible from outside, so you’ll see a lot even without the tickets to go inside.
Great coffee at Kayim coffee shop.

High on my wishlist: floating train over Pa Sak Jolasid Dam (November to January only).
I used a simcard from TrueMove. It worked immediately once inserted into the phone, and works as a hotspot. Passport required, registration process fast.
Power sockets fit both EU and US plugs.
I did not find any ATMs that don’t charge for withdrawals.
Thai lunar calendar is 543 years ahead of the Gregorian calendar, don’t be confused when you see it on posters/websites - you haven’t travelled into the future.