I don't really know anything useful about mountain bikes, but it's been in the back of my mind for years that I should get one and I could have a lot of fun exploring with it. It would get me much deeper into the mountains than my feet can, if I only have a day to roam. I'll be really curious to hear what you come up with.

But I can offer some ideas about where you can ride it:

If you're fit, take it up the Fourth of July Creek trail in Leavenworth, which climbs pretty steeply toward the sky, go right when you get to the top of the mountain, and take the Icicle Ridge trail back down. Then follow the road a few miles back to your car. You could call that one "ass kick hill" but the scenery is breathtaking, it's pretty lonesome at the top, and feels wild and primitive for most of the ride. (Bring plenty of water!)

All of the trails in the Teanaway allow mountain bikes. A really scenic and not too steep option is to take the Beverly Creek trail to meet Iron Mountain, and follow that trail back down to the road. Great scenery and you could hang out with mountain goats. Koppen Mtn is popular with the MTB crowd. There's a loop around Esmeralda Peak that's not too hard, but very scenic and fun.

Winthrop is kind of a mecca for cycling, especially on trails (but it's fun for road riding, too).