Frazier Park is in a lot of ways a hidden gem. It's very much on the outskirts of the city, so if you don't live in the Gateway area, you're probably not spending a lot of time here. But you should.
After some much-needed cleaning-up and development, Frazier Park is a great place to head now. Great place to jog or throw a frisbee with friends, and there's a decent outdoor basketball court there, too. The games haven't been that great yet, but it's becoming more and more popular, so that should change sometime soon.
Again, though, the real coup de resistance of Frazier Park is the new dog run that opened there recently, which has been an unmitigated success so far. This place is awesome and is a huge hit for all dog lovers in the city. The dog run is huge--there's lots of room to let your dog run free and play with the 50 or so other dogs that might be there, too. There's even a smaller pen where puppies and tiny dogs can frolic without getting trampled by larger pets.
Again, great place to take your dog, especially on the weekend. Great place to meet fellow pet-lovers, too.