Frisco is Summit County’s hub for fabulous paved bike paths. From the Frisco Marina you can wrap most of the way around the reservoir to Dillon (7.5 miles), for a great family trip with minimal elevation gain, and then on to Keystone (13.5 miles, 1200ft elevation gain) – or pedal around the other side and up to Breckenridge (9.5 miles, 500ft elevation gain).
Alternatively, you can head off in the other direction to climb Vail Pass (12 miles, 1550ft elevation gain) and roll all the way down to Vail. Many bike shops run shuttles up to the top of the pass if you're only interested in the downhill ride. Grab a copy of the free Summit County Bike Guide in rental shops or the Information Center.
Note that you can take bikes on the Summit Stage if you run out of steam or only want to go one way.