A couple loops to choose from with scenery ranging from wooded to open meadows to views of Boulder below. Not too crowded on weekdays and some of the trails are closed to bikes on Wednesdays and Saturdays.
From US36 in Boulder, go west on Canyon Rd approx 4 miles. Past the tunnel, turn right on Sugarloaf Rd. Go about a mile and turn right on Betasso Rd. The trailhead is about 1/2 mile or so on the left.
http://www.tripleblaze.com/trail.php?c=3&i=90