My info is 3 years out of date, so take it with a grain of salt...
Trains when ever available are always the best way to travel in Thailand(besides a motorcycle of course!). Theres no honking, no swerving, or cramped seats like you get on the overnight bus...just the slow rocking of the train car and lullaby sound of the tracks. If memory serves me right the train took about 8-10 hours and was done over night. When ever I go to Thailand, I go straight to the train tracks that pass by the airport and head north to the small town of Ayuthaya (an hours ride north of Bangkok) and avoid the bustling city of Bangkok. From there, I can arrange any mode of transport to anywhere in the country. Though, if you have never been to Thailand, its a must to spend a few days in this city of organized chaos.
Julie's guesthouse is my fav of Chiang Mai. Nice rooftop hangout, central location, and good people running it.
If you are looking to go on a hill tribe trek, don't book it in Chiang Mai. Go to Pai for this, a few hours west by northwest of Chiang mai. Treks are less disney world like what you get from Chiang Mai operations and its also a lovely little mountain town with a great night life. Its kinda like a beach life mentality in a mountain side setting. You can also rent a bike for a couple of bucks a day and explore the surrounding towns.
Hope this helps