Beth, If you can find a bike or 2, I'll ride with you up Mt Diablo on a Friday (not as much traffic on a weekday up there). Depending on the weather, we should start early as it gets quite warm up there. Today when we got to the junction of the north and south sides it was 90 degrees in the shade with little wind. The summit was probably about 95 or so. We came down at the junction due to time constraints and heat.

You can PM me if you wish and I can give you some tips on riding on BART and what station you should go to. BART is easy to get around as you sound like you may know already.