Terminal 21 Asok
A mall built as an airport — you clear 'immigration' at the door, then ride escalators between Rome, Paris, Tokyo, London, Istanbul and San Francisco.
The conceit runs all the way through. The floor directory is laid out like a departures board, and each level is decorated in full: a black London cab, a Japanese shrine gate, a scale span of the Golden Gate Bridge.
Bangkokians will tell you, unprompted, that Terminal 21 has the best public toilets in the city — each floor's restrooms themed to its city. In a hot capital with few public facilities, that is genuinely useful information.
The top-floor Pier 21 food court serves some of the cheapest sit-down meals in central Bangkok, most dishes ฿40–70, in a building where the shops below sell luxury. It sits directly above Asok BTS and Sukhumvit MRT, so you can reach it on either line.