Really depends on how much you want to spend and how far you want to travel. I try to stay out of Gaslamp because I'm broke all the time. South Park has the best beer bar in the city, Hamilton's. Highly recommended. Also, the best sandwiches in town are right around the corner at Grant's Market. Seriously, amazing. North Park I'd recommend Tiger Tiger, Crazee Burger, Sicilian Thing (for pizza), Bangkok Spices. If you like Russian food there's a place called Pomegranate which is fantastic. Rabbit stew is killer. There's an Ethiopian place next door which is great too. I would highly recommend taking the time to visit Carnitas Snack Shack if you feel like being in cholesterol heaven. Pretty much any taco shop you wander into will be good, but you already know that. Favorites are El Zarape and Saguaros. If you make it up into my neighborhood you should probably eat at Zia Gourmet Pizza. I live right upstairs from the place and literally eat there 3-4 times a week. It's probably why I'm so broke all the time.
As far as fun shit to do, Balboa Park. There are several lawns where you can bring canned beer and hangout in the sun. All the museums are pretty phenomenal although somewhat overpriced. The zoo is a standard, good time. If you feel like exploring a bit Coronado is beautiful and it's work walking around for an hour or two at the beach there. Any other specific things you're looking to get into? I can probably point you in the right direction.