Hmm...most of the restaurants on Harbor Island (where the Sheraton is) are more up-scale...there's a place called Pizza Nova not too far away, though, and that's good - wood-fired pizza and pasta and salad. It gets pretty busy and noisy, though, but the food is good and it's a casual setting.
Further away (15-20 minutes from Sea World/Zoo) is a place called "Dave and Buster's" -
www.daveandbusters.com - which is a big arcade/midway games restaurant place. I've never been there, but I know a lot of people seem to like it.
There's lots of Mexican food places in Old Town, though most of them tend to be run-of-the-mill Mexican food - nothing too special about them, though the atmosphere in Old Town makes it an interesting place to walk around.
There's lots of chain restaurants on Point Loma - ~5-10 minutes from your hotel - Denny's, Coco's, Olive Garden, Chili's, Red Lobster, etc.
There's an Italian place called Filippi's in Little Italy (~10 minutes), and it's a place that's been there for 40+ years - at the entrance to the restaurant is an Italian grocery store. Red-checked cloths on the tables, straw-wrapped chianti bottles hanging from the ceiling - it has atmosphere! More authentic pizza and toppings (and they top their pizzas with a thick gooey layer of mozzarella cheese!) - you won't find barbecue chicken or Hawaiian pizza here! The food is good, though.