I was going to suggest Koh Kred as well (as Jorgen mentions) since it's so close to BKK. I've not paid the 1200-1500 for the return boat, but know that some do. I've been fortunate enough to pay much less, but don't count on it as a foreigner.
You may also like Pi Mai or Buri Ram for the Khmer-style ruins, depending on your schedule.
I like to drive to Mae Sot from BKK, but your time may be a bit tight. And one of my favorite areas is Kanchanburi province, but not so much the typical "Bridge on the River Kwai" stuff (actually River Kwae), but Sangklaburi at the border with Burma/Myanmar. It's about a half-day, pleasant drive from BKK and you'd want at least a night or two there. Look to stay at P Guest House or Ploy Palin Resort. Nothing fancy there, but these two spots are pretty good. The food at P Guest House is good, so I often stay at Ploy Palin and eat dinner at P Guest House. (I don't eat at Ploy Palin. It's probably fine, but not my cup of tea.)
I don't see where you mention the month you plan to travel, so that could have an influence on suggestions.