Big Basin State Park is one of the best places for trail running in the Bay Area. The park is most recognized for its spectacular ancient redwoods, but the 18,000+ acre park also features an expansive wilderness with several beautiful waterfalls and gorgeous vistas of the Santa Cruz Mountains and Pacific Ocean. I have run most of the trails in Big Basin State Park and I plan on coming back to run them again! Here is a map I made of some great loops in the park and links to my blog posts on Big Basin:
Leor Pantilat's Adventures 
Thanks for the great post, Leor. I agree completely. Unfortunately, I don’t live in the Bay Area any longer but when I did Big Basin was my favorite place to run. The drive from San Jose was always worth the experience. Those are truly great trails year round. I was even fortunate enough have an amazing mountain lion encounter in the park.
Take care.
Chris
Thanks for the post and the many great photos you have posted of the park.
Big Basin’s trails are awesome and offer some simple camping options, especially during the off season. I can drive down from San Francisco on a Sunday, hike all day and camp the night while still making it back to the city for work on Monday (after a run through the redwoods of course). Not bad.