I have just spent part of the day at Golden Gate park in SF. We went to the Japanese Garden and the Botanical garden across the street. Both were really great.

The Botanical garden is free and one of the best collections of plants, shrubs and trees I have ever seen. I have recently been at both Longwood and Kew, and to be honest, I like the SF Botanical Garden at least as much and maybe even better. I was disappointed with Longwood.