This used to be the Del Valle State Recreation Area. The primary attraction is Del Valle Reservoir - a popular place for boating and fishing. More important is the access to the Ohlone Wilderness Trail. Follow the trail for 20 miles to reach the Sunol Regional Wilderness , or just climb the extremely steep bit to reach Murietta Falls (or in summer, the Murietta Dry Streambed).
There are two campgrounds. Del Valle Park is at the southern end of Del Valle Reservoir and mainly provides access to a boat ramp. Vendanos campground is only accessible by boat or trail, and sits on the eastern side of the reservoir. Water and toilets are available at both campgrounds.
Reservations required in advance - call 510 562-2267.
USGS 7.5' Topo Maps
East Bay Regional Park District