We're not generally mountainbikers (we prefer to hike our trails) but Rancho San Vicente, Calero County Park, seems to be a great place for new and seasoned moutainbikers alike. The trails are wide and even, the grade is a steady climb, and the park even clearly marks part of the trail as one-way for everyone's safety - hikers, bikers, and horseriders. There are plenty of under 10s and most cyclists are good trail citizens traveling at safe speeds (not always the case in the bay area). And the views are spectacular. Yet we would still recommend keeping a careful eye and ear out for fast approaching two-wheelers.
This cache is a bison tube hidden in a typical fashion, right by the trail. Please note park hours.