My girlfriend and I visit Mount Diablo State Park last Month.  It was a bit hot but really a good day out.  You can drive to the summit and hike around at the top or you can stop a ways down and hike your way up.  We did both!
                                

Gates open at 8 a.m. and close at sunset.

Mt. Diablo State Park boasts amazing views from lofty heights with its 3,849 foot summit. The park offers hiking, biking, horseback riding and camping. There are numerous trails for all levels of experience. As well as some pretty great spots near Rock City to get in some rock-climbing. It does get pretty hot here in the summer (mid 90s), or basically anytime you are planning on hiking a mountain, so bring plenty of water!! I found that a camelback works best as there are not a lot of stops along the trails to pick up water. If you come during the spring, the temps are cooler, and you can see some of the areas most beautiful wildflowers.

Geographers claim that hikers can see more of the earth's surface from the top of Mt. Diablo than from any other peak in the world with only one exception: Africa's legendary 19,340 foot Mt. Kilimanjaro. A relatively easy way to the top is via two mile round trip Juniper Trail. If you're feeling more ambitious, you can tackle the 6 mile round trip Summit Trail. And yet another great way to tour the park is to follow The Grand Loop, a seven mile circuit that connects several trails and fire roads and offers views of, and from, Diablo in every direction.