Hiking was mentioned in the original post, an activity that I would rate as higher quality than the climbing in SE Asia. Jungle adventures are cool. Get a good machete (the best one has small forward curve at the tip and is sometimes called "garden knife"), a file, an okay map, and a jungle hammock if you're going to stay out; add a GPS + compass and get after it.
The hiking in N. Thailand is crappy and everyone gets a bit jumpy if you roll around the hills solo. Some places are okay but I've seen a lot of weirdness up that way and would give it a miss (getting chased by a german shepherd springs to mind...) Walking around the islands in the south that people actually live on is cool. Usually, only about half the beaches are accessible by road and then only by boat for half the year. They all have trails between them that are usually pretty overgrown. Hit it in the off season, sleep at the inhabited beaches and walk all day alone to the next one. East Coast of Koh Pha Ngan is very good and takes about a week. In Laos, hiking is good almost anywhere-just hop on a truck out of town and when you see a nice valley hop off and find a trail. The north is really cool but the jungles are dark and scary and there's lots of leeches. Hmong villages in Xieng Khong province are worth visiting if you can find out which ones they are; they're quite different to the so-called Lowland Lao. UXO is a problem out there so it's probably best to have a guide, the best of whom is Mr. Manophet in Phonsavanh. Around Sam Neua is amazing but you'd probably want to have learned some basic Lao, have a high tolerance for rats, and love 18 hour windy mountain bus rides while sitting on a bag of garlic in the aisle watching ear-splitting Thai soap operas.