Camp Hale is great. We like to go up Webster Pass. Lots of cool trails up that way. But you have to go late summer because it can be blocked by snow. Also, check out Crystal Mill. Neat drive.
ok, ok...I like the Georgetown & Central City area for close stuff. Gonna wheel the Ouray & Telluride area this year, I'm sure I'll love that area too.
Definitely Georgetown and Central City. Do yourself a favor and find Charles Wells' books on Colorado 4x4 Trails, and wouldn't be a bad idea to hook up with some of the local 4x4 groups, whether an in-person group that has to have meetings at set times and places, or an informal like Colorado4x4.org - also check out TrailDamage.com (damage from the trail, not to the trail) for other ideas on what's available and the difficulty.