Green Mountain has a lot of trails, I can spend a half a day there....Im not very good tho. I love Elk Meadow in Evergreen, some great single track along with some brutal hill climbs with a great view from the top of Bergen Peak.
Another great area is White Ranch Park (west of 56th ave and Hwy 93). Tons of trails, also with some great hill climbs. The Ranch has two lots, one at bottom and one on top if you dont feel like beating yourself up. Have fun!