In search of adventure in Thailand

Alexander, I have 2 suggestions for you, both based on my own experiences. The first if Flight of The Gibbon, this is a full day adventure zip lining through the canopies of rain forests, I did it in Chonburi but there are several locations, including Chiang Mai and Phuket. 

My second recommendation is a little more gentle, but one of nature's unique sights, a sight most people ever get to witness on Nat Goe or Discovery Channel. The great sight I'm referring to is that of millions of bats leaving their cave to hunt. This takes place at Khao Lak Chang Bat Cave in the Pak Chong district. You could combine this with a trip to Khao Yai National Park, or make an excursion from Bangkok.

Try White Water Rafting in Khek River, Phitsanulokprovince. It isn't the top destination but challenging.

Among white water rafting spots in Thailand, it is the hardest with 5 levels which other spots can not compared. 

The trip take 2 hrs, 8 cataracts along the natural landscape of central-northern Thailand where not too touristy for Western travelers.

The best time is rainy season.

See pics from my camera. This is at the beginning of the adventure.