A wonderful 2.5 acre garden worth visiting in Palo Alto, CA is The Gamble historic Garden. A free, self guided tour takes about an hour and there are benches and many places to sit and enjoy the day if you have the time. You can also head inside the Gamble house, we found a nice little horticulture library within.
A bonus to the location is that it is in beautiful old Palo Alto. The homes and gardens in the area are unbelievably beautiful, no expense spared!