I used to bike up to Skyline on Old La Honda road. I didn't consider it to be really impressive nature... at least not compared to the above mentioned cities. IMHO there are too many people and built-up land there (a general problem in the Bay Area), and whatever hikes/etc. are available are flat and rather mediocre. Marin is not bad but also crowded, flat and gets old fast. Places like Skyline, Saint Helena, Mount Diablo, Half Moon Bay, Pinnacles etc. are really only good for 1-2 visits and then to impress visitors :)
Sierras are amazing, but far away (both Rockies in Denver and Cascades in PNW are much closer and the traffic is not so bad), and the ocean is very cold, you have to be really psyched to surf or do anything like that in the Bay Area.
I mean sure it's better than most places East of the continental divide, but not particularly great. Especially with all the people :)