Where's the best place to go mountain biking in Boulder?

In North Boulder, there are some nice rolling/level trails (the Foothills Trail) around Wonderland Lake (just to the west of Broadway & Quince Avenue) that run northward along the west side of the Dakota Ridge Neighborhood. This same trail (single track) then dips under HWY 36 heading east on the very north-end of Boulder out to Boulder Valley Ranch Open Space area with numerous loops that head eastward towards Boulder Reservoir.

In South Boulder, there are some nice trails located along Marshall Mesa, which is located on the east side of Broadway/HWY 36 at the Eldorado Canyon turnoff. There is a nice single track loop on the north-side and top of Marshall Mesa with a connecting trail crossing HWY 36 and heading west towards Dowdy Draw that eventually links up with the Flatirons Vista Trail.

Up above Boulder, there is a more technical single track trail at Walker Ranch Open Space Area. Head up Baseline, which climbs Flagstaff Mountain past the amphitheater, towards Gross Reservoir on the back-side of Green Mountain. Eventually there is a trailhead on the south side of the road for Walker Ranch Open Space, which features a technical loop trail trail that heads down to South Boulder Creek.