Mt. Diablo is a great place to hike, combining great vistas on the south side with verdant trees and streams on the north side. The Circumnavigate MtD series combines the best of both sides. Don't just stand there -- come hike these great trails! There is an entrance fee of $10.00($9.00 for Seniros), but it is free if you have a California State Park annual pass.
The MtD Grand Loop series goes around the summit on all sides, in a roughly rectangular pattern, covering some of the loveliest areas of the mountain. A good starting point is the Juniper campground (GCM61W, at N 37° 52.611 W 121° 55.848), from which the Deer Flat trail heads north to Deer Flat, then on the Meridian Ridge trail to the Bald Ridge Trail, then south to Prospector's Gap; then south on the North Peak trail to Devil's Elbow, then on the Summit Road uphill for a short distance to the Juniper Trail, then back to Juniper campground. The entire circuit is 11.4 km (6.9 miles), with a total of 615 meters (2000 ft) vertical climb.
This cache is located along the North Peak trail, at a fun spot in the "Circle of Life".
WATCH OUT FOR POISON OAK, BOTH ALONG THE TRAIL AND NEAR THE CACHE.