I just hiked the Oursan Trail, on the north side of Briones Reservoir, last Saturday. This is a really nice, up-lifting hike in the fall when it’s a little cool (60’s) and you want to be in the sun. Just make sure the trail has had time to dry out from any recent rain and isn’t too muddy. You’ll love all the views of Briones Reservoir shimmering in the sun!
EBMUD is considering opening up some of their trails, like Oursan, to mountain bikers. Here’s a link to a recent article in the CC Times.
Click here for a full description of this hike. Remember to bring your EBMUD pass.
This is going out about an hour ( 3.1 miles in this case) and turning around for a total of 6.2 miles.
The Undiscovered Hikes of Lamorinda 
Well you mountain bikers need to know that only 2 trails will be opened to mountain bikers in the near future (pending adoption of the new EBMUD Master Plan in the very near future. The trails are the Eagle Peak and the Pinole Valley.