Dolores Park (especially the slackline trees) and that really good ice cream place with the line that wraps around the corner...
Stern Grove in the summers
Crissy Field anytime, but especially on a sunny day
The Golden Gate Bridge
The Mission, particularly the intersection around Valencia and 18th/19th...
Mission Cliffs Climbing Gym
But there are SO many other cool places...any other major favorites that I missed? I need more recommendations...