Really try your hardest not to overpack! 7/11s are everywhere and you can get absolutely everything you need there. However, sunscreen is really expensive in Thailand, so bring a lot of it. After paying $1 or $2 for everything else, shelling out like $13 for a tiny bottle of sunscreen will be painful. Also, make sure to bring tampons if you're particular about the kind you use. They're difficult to come by in Thailand, and when you do, they're usually some uncommon brand.