Courtney and I recently took a trip for our 1-yr anniversary out to summit county. If you didn't know: Summit county is awesome in late september because the aspens are amazing: Also, everything's cheap because it's the off season: from being able to get a condo for $50/night on airbnb to 1/2 off entrees at many area restaurants.
Also awesome: Courtney got a mountain bike. A hardtail 29er that we got off craigslist. While it needs some upgrades (if anybody has an extra pair of 29er wheels or fork, leave me a comment), she shreds on the thing.
Unfortunately the only trail I've ridden in the area is the Peaks trail (which is amazing, but not easy). Courtney did awesome on it, although I realized she'd have some more fun if we road easier trails.
I stumbled on mtbproject.com and it saved the day. The site is awesome: it has lots of beta on area trails with user ratings, and they're ranked by difficulty. Plus, if you grab the app, you can download trial maps for offline usage. We did Keystone Gulch and Soda Creek, and had a great time. I would def. recommend for anybody who likes mountain biking and exploring new areas.