A tranquil 55 acres inhabited by 7500 hundred plants, the garden is free and delights many. Guided walks start daily at 1:30 - on warm days, picnicking is a great option too.
Within Golden Gate park and close to the Cal Academy of Science and de Young Museums (and just a bit further to the Conservatory of Flowers), the Botanical Garden combines nicely with other adventures.
Address: 9th Avenue at Lincoln Way, SF, CA
Website: http://www.sfbotanicalgarden.org/
Wikipedia: http://en.wikipedia.org/wiki/San_Francisco_Botanical_Garden