For Bangkok:
Siam area is basically a city area with malls, high ends stuff. No real reason to spend extended time there because I'm sure there is no different doing that in your own country.
IF you are really interested in malls,I will recommend Terminal 21 which is at Asok BTS.
Every level is a different themed airport terminal. Even the washrooms are special and worth a look see at different levels. Feels pretty special for first time goer imo.
If you are there on the weekends,
try Chatuchak weekend night market. It opens around 5pm but all the stalls are only fully up around 7pm. It's basically a flea market of sorts but is ridiculously huge. If Chatuchak bores you, there is another night market with more eateries 5mins walk from there called JJ Green which just open less than 5 years ago.
Both are walking dist, 5mins or so, from Mo Chit BTS.
Another alternative is Chinatown of Bangkok.
It's a whole stretch of road that will take you about 45mins or so to stroll down. Plenty of Chinese foods to eat, some dry products like dried mangoes, bird nest, abalone to take a look in the stalls. There are numerous food stalls to try as well. I will avoid the one with the red and green chair that sells seafood and stir fry vegetables though. They are overpriced and tourist targeted. You will recognize these two stalls easily when you see them. Go to the quieter side, deeper in the alley for authentic Thai Chinese foods like fried rice, pork trotters and all for less than 100bahts per dish.
Chinatown will requires you to take Uber/Cab. Leaving the place back to your hotel after 23:00 might be a little difficult though unless you are willing pay fixed price for the cab fares, usually 250 ~ 400 bahts. But if you went by meter, it will probably cost 150bahts back to city area.
IF you want a more laid back, drink and chill place. I would recommend Ari area. Ari BTS
Not exactly easy to navigate but there are plenty of live bands bars in that area, it's more for the white collar locals and occasional tourists. You won't see as many tourists there, but the waitress there should know better English to help you out compared to other areas.