Are you planning on eating inside or outside Petco?
If you are planning on eating outside, my DH says The Tin Fish is a good restaurant:
http://www.thetinfish.net/right-tinfish.php?nav_ID=106&nav_Parent=104
Inside Petco Park, there is a good selection of food. Last season, they had Rubio's, a Mexican Food place known for their Fish Tacos, Oggi's Pizza, Anthony's Seafood, and a BBQ eatery. I believe there's also a sit down restaurant inside Petco Park, but we've never tried it.
HTH and have fun!