There is a mile long trail along Horseshoe lake for anyone you're just introducing to moutain biking. You can ride it if you're experienced, but you're gonna be bored. Also keep in mind that there are paths around the lake that are hiking only. Please do not bike on these. Watch your speed too, as there are lots of hikers and strollers around the Lake.
This is better riding on this trail though. If you go South from the lake on the Ridge trail, there is a 3 mile one-way ridgeline singletrack ride that connects with Long Ridge which makes a great singletrack ride that's not too crowded. You'll start out on a fireroad that goes past a Christmas tree farm, and then becomes some nice rolling singletrack until you reach Long Ridge.
If you go North of the lake, you can ride various trails that will connect you with Russian Ridge and Monte Bello Openspace areas.
Take Highway 9 out of Saratoga up to Skyline Blvd (Hwy 35). Take a right (you'll pass Saratoga Gap trailhead). You can park at Long Ridge, take the entrance trail down, then take a right to the ridge trail.<br /><br />The main entrance is a few miles past that, with a rather large parking if you don't want to risk it by parking on the edge of the road. There are also facilities at Skyline Ridge.
http://www.singletracks.com/item.php?c=1&i=871