Mt Diablo, Bay AreaMt Diablo (1178m) is located in the East Bay, northeast of Berkeley. (Note that all the maps and the trail signs are still in ancient "miles" and "feet", which probably means that they are very, very old and very, very inaccurate).
Directions to the trailhead: Take the Ygnacio Valley Rd exit of 680. Go east on Ygnacio Valley Rd for about 10 minutes. Turn right into Clayton Rd. After about five minutes, turn right into Mitchell Canyon Rd. Drive to the end. Pay for parking ($6 in 2015). The trailhead is near the visitor center/ ranger station. There is a ranger station that has maps. The maps are better than nothing, but they miss some trails.
Behind the visitor center look for the Mitchell Canyon Trail. You need to take an immediate left or you'll end up on Mitchell Canyon Rd. Follow the trail through the grass fields. Turn right into the Eagle Peak trail. This trail winds its way up to Eagle Peak. It takes about two hours to Eagle Peak. Then the trail winds down to the Meridian Ridge Road (Murchio Gap) where you hit a dirt road. From this point all roads lead to the top but best is to cross the road and continue straight up on a narrow trail towards Prospector's Gap. Both the road and the trail meet at Prospector's Gap with the North Peak road. From Eagle Peak to Murchio Gap takes about 15' and from Murchio Gap to Prospector's Gap takes another 45'. You are 45' away from the summit. Turn right into the North Peak trail (here the name might be confusing because it does not go to the North Peak but to the real summit). After a few switchbacks, this trail emerges on the other side of Mt Diablo (the southern side). At this point you have a much better view of the summit tower. You can either follow the trail that goes towards the paved road, the lower parking lot and eventually the summit plaza, or simply start walking straight towards the summit tower (there are use trails but not easy to follow).
From the bottom to the summit takes about 3h 45'.
Returning straight to the Mitchell Canyon parking lot is best done by returning down the North Peak trail to Prospector's Gap and turning left into the Meridian Rd (just to test one of those very steep dirt roads). Always take left turns until you return to Murchio Gap, the junction of Meridian Road with the Eagle Peak trail. Just before that junction there is the junction with Back Creek trail, a steep trail that goes straight down to the grass fields. This is possibly the most interesting (and greenest) trail. Once down, take a left on the Bruce Lee trail and Coulter Pine trail to get back to your car. Downhill along this route takes about 2h 30'.
The grand total is 6h 15' of actual hiking.