Pretty much whatever the fuck you want. It's hot so make sure they're summer clothes, but also have a few longer bottom-half alternatives (skirts lower than the knee etc.) in case you visit some temples and shit. The Palace tourist site in Bangkok makes a bit of money off of tourists who are refused entry for their clothing and have to buy from the local shops. If you're not going to really nice clubs then you can get into most bars and clubs with pretty much anything on.