a great place to visit, just a few hours out of Bangkok (more if by bus). so it’s not an epic to get back to bangkok in the evening the next day—a good weekend trip, popular for people from bangkok.

I like a beach to have hills in the background so a few hours more (or a quick flight from Bkk)then your in Koh chang
http://www.43places.com/places/view/199501

This website (not mine) gives a bit more ideas about ko sumet:
http://www.bugbog.com/beaches/beach_pictures_thailand/beach_pictures_ko%20samet_2.html