So a few weeks ago, I traveled to Santa Cruz and the Moaning Caverns out in Calaveras County. The caverns are about a two hour drive from Martinez, so relatively close. If you've never been into any caverns, I highly recommend it!
(Although the best caverns are still Carlsbad :P)
Anyway, if you haven't been to the Moaning Caverns, I highly recommend it. There are also two other caves you can visit within the area, the Black Chasm Cavern and the California Caverns.
Thanks for reading!