For someone new this one is probably best top roped off the tree (theres two hangerless bolts up there too) but this chimney is a very good learning opportunity. I would not have a beginner lead this one, the gear and climbing is a bit awkward for someone with little experience.
Another hand crack in a corner. Top roping is best, I lead it and ended up doing a silly traverse right to get to a bolted anchor, I think going straight up is 5.10a?
I've heard there's good cracks a summit rock at castle rock, but I have not been to check it out.
Beach Crack - 5.7 - Short, sandy crack, but easily protected. Makes a good lead for a newer climber.
Chouinard's Crack - 5.9 - Funky, flaring wide crack that can be jammed using a number of techniques. Last 1/3 of climb is unprotected face climbing in a seam, but easier.
Mt. Diablo's best cracks:
Diagonal Crack - 5.10c - Longer, left leaning crack, often with good jams. Climb through sandy pods, sometimes with questionable pro. A nest of bats lives deep in the crack, half way up this climb.
Ozone - 5.10c - My personal favorite. Lieback and chicken wing up the initial offwidth crack. Pull a roof into a hand and finger crack above. This thing felt like a massive sandbag before I knew how to jam. Now the grade feels about right. Makes for a very satisfying lead.
In Oakland, near Lake Temescal, is the Golden Gate Wall, a long stone masonry traversing wall. Worthy in its own right. Just uphill from that is a 30 ft concrete expansion crack. Easy to walk up top and top rope off the railing. Tight hands, 5.10. Slippery feet.