you are squeezing in a lot in 6 days, if leaving from bkk, actually 4 days, and 2 for travel. if flight not set, i would go to phuket, kata beach has some surfing, climbing would be possibly krabi, actually couple hours away, on the way, phangna bar for kayaking, though a bit weak. hiking and biking is easy anywhere, snorkeling, maybe, but probably an all day affair, and also a bit weak. as said, if flight not set, should be able to get something out of phuket.
culture and buddhism, wait till next trip. good luck. if flight out of bkk to bali, maybe a trip to kanchanaburi, though no beach activity, though the rest might work.
