We took Hobie to Pine Lake last weekend. It's right off Sloat, and it's surprisingly quiet. There were a lot of dogs there, despite the dog run being under renovation.
The dog area seems huge, and it's one of the biggest dog parks I've seen in San Francisco. Dogster has directions and the address.
It sprinkled a little bit, so we had to deal with a muddy, but happy, dog.
There are a few short trails behind Stern Grove, and Hobie had a great time running around there. For the humans, though, the walk around the actual lake was nicer.