I took a Tai Chi class in college and found it stressed me out instead of relaxing me -- I just couldn't remember the order of the moves in the form, and found some of it just too weird. (Tossing rice at monkey??)
For stretching and toning, I like Pilates so far -- I have the beginning mat workout by Ana Caban and it is pretty good for starting out. I've also tried Yoga, which I find much more relaxing -- the workout book I have is very easy to follow and doesn't involve too many weird moves. I'm just bad about making a routine of either.