completely overrun with tourists, although there will be more language barriers in those places. I'd recommend Krabi, it's a pretty good balance. I would also go up North to Chiang Mai and do some of the jungle stuff up there. I'd probably say 3 days in Bangkok, 5 or so in the North, and 5 or so in the South, although you could tweak the North and South if one appeals more to you than the other.