Chatuchak market is a must visit! They have it on the weekend and it is one of the largest market I've ever seen. You can buy anything from furniture, pet, plants, and of course fruit.
Or Tor Kor market is a fruit paradise too!
Fresh fruit everyday for cheap price!
Just google for location.
I also walked around the city at night in Bangkok, it is relatively safe and you can always find street vendors selling fruit or fresh coconut everywhere!
Any street food vendor in Bangkok serves food that deserves 5 Michelin stars, in my opinion. Everything was so delicious and fresh.
Enjoy!