My neighbors in Bernal Heights don't tend to their backyard which is fine by me because it means more blackberries for the picking. Right now, their vines are reaching over into our yard allowing me to get a good cereal-bowl full of berries in the morning for breakfast.
While berry-picking season is on the brink of being over anywhere that actually sees the light of day in the summer (Marin, Sonoma, etc), in SF where the sun has been buried by the fog, berries are still to be had on Bernal Hill (there's a huge patch at the gates of the bottom of the hill), Glen Park, in the Presidio and Golden Gate Park. SF Weekly also reported Tank Hill as a good spot. Berry season is fleeting, so hurry. And should you miss the berry boat, or get shoved out of the way by a small kid eating their way through the brambles, there's still the farmers market. (Tip: In the city, I advise you aim for the blackberries that grow high. You never know what goes on with the blackberries down low.)
My favorite fruit crisp recipe, a classic from Marion Cunningham, actually happens to be the easiest recipe on earth. Without nuts or oatmeal or pretty much anything extra, it's not fancy, but it's all-American, good and sweet (perfect for tart berries) and forms a nice crust. Find it here on Saveur.