If you like hiking, the San Francisco Bay Area is a great place to be. There are tons of trails of varying difficulty. Being along the coast, there's not really a massive amount of elevation gain from sea level, but there's still decent trails to be found.
In what is arguably just outside of South Bay, Castle Rock State Park in Los Gatos offers sprawling green forests along the crest of the Santa Cruz Mountains. A small tip about hiking in the bay area - go early to avoid both the mid-day heat as well as to nab a parking spot, which can be challenging at more popular trails.
Taking a breather to snap this photo.