mt diablo has great views, so worth the trip, but by and large is not great mountain biking. long climbs followed by fire road descents. there are a handful of legal single tracks that are ok. Dan cook is a fun short single track towards the bottom (go down on the right, the left is not as fun). Mothers is cool because of the great views and is quite steep, but otherwise non tech. there is a longer single track off curry point which is fun, but drops you off way out towards livermore and is a long pedal home. There are some more fun single track trails, but they are all illegal and i would not be able to describe how to find them. this time of year the single track is also quite bad with poison oak and stinging nettle. for the fire roads – wall point down dusty road to pine creek is a nice stretch which if ridden fast can be challenging and has some creek crossing at the end. good luck!