Train Hard - Stay Humble
If you want to train in the Mission, this discussion is the place to find a great spot. If you want to share a cool spot, this is the place to add it!
My contribution: The park(s) at Potrero and 26th street are pretty good. One is a playground that seems to be pretty empty around 19:00 and later, the other is a fairly busy place with a skate park, regular football (soccer) games, and another playground. The playgrounds aren't necessarily the greatest, and the skate park is usually busy, but there are three walls of varying height at the south end of the park and some picnic tables in two different places that might be helpful. Also, there are some obstacles on the outer edges of the skate park that a traceur might reliably stake out without being ambushed by skaters.
Haven't checked out Potrero Del Sol park yet but would like to explore that area and see these progressive walls sometime. The new playground, and other areas of Dolores Park are very traversable and fun to play at as well. Then there's Mission Playground (Valencia between 19th and 20th), another new park with construction wrapping up soon that so far looks like it may have obstacles suitable for training.