Mauna:

I believe there may be one in Mission Valley area on El Camino De La Real street, I have seen, several years ago, some young high school boys head down to the south side of Tecolote Canyon and do some BMX riding on the BMX hills kind of in the middle of the south Canyon area (not sure if they (the BMX hills) are still there. I would also sign up on a cycling website so you can socialize and find out where the hot spots for BMXing are located.