Great bike trail right in the Denver metro area. 30 minutes SW of my place, past Chatfield State Park, lies Waterton Canyon. The canyon ride along the river gets more interesting as you head farther up; it could be a painful ride in direct sun on a warm day. Once you make it up to the Colorado Trail junction, the pines and deciduous tree provide ample shade. Did this on a cloudy Saturday morning 8:40-11:20am.
6 (light uphill) + 4 (somewhat technical variable terrain) + 6 (light downhill return) miles = 16 miles in ~ 2 hours 40 minutes