This isn't really one location, but is a string of parks along the hills between the San Francisco Bay and the suburbs. The one I go to most often is Tilden because of the location. In Tilden park there are trails, a merry-go-round, a lake for swimming, steam trains, a "little farm" with animals, and a botanical garden. My favorite trail is the one to Jewel Lake because of the boardwalk though the marsh. Inspiration Trail has great views. Other parks in the park district such as Lake Temescal also have swimming and lakeside hiking. Briones has fantastic hiking as well. There are some attractions for small children, some for teens and adults -- you might want to go to the website and see.
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