I've been to Thailand and Singapore many times, but not since covid, so my info might be out of date/changed.
Bangkok- Pantip plaza was a multistorey electronics/computer/gaming shops. At the bottom were whole computers, cameras, games etc and the further up the levels you went- it was more the actual components to make a computer from scratch. If you teen is into gaming or you are looking for a camera etc its worth a look.
Right next door is a multistorey clothes/shoes/accessories centre called Platinum. It used to be 'wholesale only' but often if you bought 3 things from the same shop, it was fine. Some just allowed you to buy 1 dress or 1 top etc. I'm a UK size 6 shoe, but rarely found any shoes that would fit my massive, western hoof!
Koh Samui- Had the most amazing, laid back and outdoor airport! It was very small and the waiting areas etc had a cover- but you could see grass and palm trees etc, like you were waiting in a garden for your flight. very relaxing
How old is your teen? A boat ride away is Koh Pha Ngan, famous for their full moon parties, mushroom omelettes, red bull vodka drunk out of buckets and more!
Singapore
The MRT is a clean, cheap and on time transport system. It has air con too! English is the franca linga, so everything has signs in English and everyone speaks it. Older people, in more remote locations not so much though.
Mustafa Centre- In Little India, its another multistorey store with all manner of things. Jewellery, toys, home wares, touristy items, clothes/shoes, a cafe, foreign exchange etc. Prices were incredibly good. I bought some lovely serving trays that were just so unique and also earrings there before.
The Zoo- Its world renown, large enclosures and have a night safari too. I'm generally not a massive zoo fan, but it really was interesting, especially seeing the range of animals that you may not see elsewhere
Jurong Bird Park- Near the zoo. It says it shut until end of 2023 due to renovations. I enjoyed it was a young teen, but depends if you are into exotic birds
Bakwa- Is like a sweet, salty, sticky, soft jerky. Its sold all over either in square sheets or round 'coin' shapes. You will see shops with it stacked up behind the glass counter- and often a long queue. Its worth it though if you eat meat.