Don't miss the weekend market, Chuktuchuk (Sat/Sun only) in Bangkok. It's hugh (size of a football stadium) and very Thai but will be hot, humid, crowded but loads of fun. Everything under the sun is sold there. Take the skytrain there.
I would stay at Pratumwan Princess Hotel (very clean, 4 star, was about $60 US a couple of years ago) with a wonderful swimming pool. It is right next to a hugh indoor, 10 story shopping arcade where all the young thai hangout. The entire 11th floor is a foodcourt. It is 5 min walk to skytrain. The area does not have night life but then you will be away from all the strip joints and go-go bars. But then, those will only be 5-10 min taxi ride away.