Trekking in the North

I haven't done a lot of trekking here, but I know a bunch of (sportier) people who have. There are a lot of places in northern Thailand where you can trek from... actually pretty much anywhere up here has trekking (Chiang Mai, Chiang Rai, Pai, Mae Sariang, Mae Hong Song, Nan, etc, etc, etc...). Chiang Mai is probably the easiest to get to/leave from if you're pressed for time. There are all sorts of different treks that you can do, ranging from 1 day to 1 week and from easy to hardcore. Many involve going to a hill tribe village, rafting (depending on the time of year) and elephant riding.

I know a good local tour operator (family business) who I've sent quite a few of my friends to and they've all been more than happy with her. It's called Destiny Travel Service and you can find them online at http://www.destinythailand.com/. They do tours all over northern Thailand using Chiang Mai as the starting point usually.

Because I'm up in Chiang Mai, I don't know of things that start and end in BKK, but it's easy enough to get up here on your own. I really love the overnight sleeper train. It's quite a relaxing experience and 2nd class is more than nice enough and very reasonably priced. Make sure to try and get a lower bunk, even though they are slightly more expensive, as they are much more comfortable than the upper bunks. The buses are cheaper, but to me they lack the romanticism of the train (plus the fact that half the bus drivers are drunk or hopped up on speed). If you're pressed for time, there are now plenty of cheap and reliable flights between CM and BKK (try Air Asia and Nok Air).

Basically as far as trekking in Thailand goes, I don't think anyone would have trouble finding something to suit their needs... it's usually more about narrowing down all the choices.