3. Del Valle Regional Park
The East Bay Regional Park District operates 65 parks covering over 100,000 acres and 1,150 miles of trails that hardly anybody outside of the Tri Valley region knows about. My favorite park of the bunch is Del Valle Regional Park, set deep in a valley framed by oak-covered hills about 10 miles south of Livermore. This dog-friendly park has everything you could want for weekend outdoor getaway (swimming, boating, camping, fishing, biking), including dozens of miles of hiking trails such as the Ohlone Wilderness Trail with 28 miles of scenic back country trail.
I also highly recommend spending some time hiking through the 5,271-acre Pleasanton Ridge Regional Park, one of the prettiest parks in the East Bay that straddles a mountain ridge with postcards views of Pleasanton and Livermore Valley. The multi-purpose trail system is also popular with mountain bikers and horseback riders.