A working cattle ranch that has multiple trails - some are open to bikes. Combination dirt road, doubletrack, and singletrack. There is a lot of singletrack along the dirt roads. One moderate climb/decent with waterbars. Good views of the plains, mountains, and Boulder Valley. From the Foothills trailhead, take Eagle Trail to Boulder Reservoir. Turn around, and connect with Sage Trail, back to Eagle Trail and return.
From Boulder, go north on US 36. Continue past Jay Rd. for 1.5 miles, and take a right on a dirt road. The Foothills trailhead is just a little farther on the left. There are several other trailheads too.
http://www.singletracks.com/item.php?c=1&i=1886