Sugarloaf Ridge State Park, Santa Rosa
This is a great park for hiking and even camping, with great views from the top of Bald Mountain, just over 2700 feet. I have provided more details in my page for the park itself, but it makes sense to describe it on the Santa Rosa page since Santa Rosa is the closest town to it, not counting Kenwood, which is tiny.
There is camping and plenty of hiking over miles of trails. A hike up to the top of Bald Mountain and down again, with a stop at Pony Gate Falls (especially nice to cool off in hot weather), can be a good day-long trek if one takes longer routes. Pony Gate Falls is a nice 25-foot waterfall cascading over rocks and, shaded in trees, is a pleasant place to relax in either cool or hot weather.
For more information, see my Sugarloaf Ridge State Park page.