I will be there for a conference in a couple weeks. I have never been. What must I do or eat while there? I am in meetings every day until 5 so keep that in mind...
If you like fish, then you need to have fish tacos. The best place is reportedly South Beach, in Ocean Beach (neighborhood of SD). I don't eat fish so I can't verify. Re: Mexican food, if you're going to be at the Convention Center, you'll be right by a trolley stop. You can take the trolley to and from Old Town if you'd prefer.
I recommend checking out Balboa Park and the restaurant there, The Prado. You could go for post-conference happy hour one day. The bar has great food and drinks and is reasonably priced. You can then walk around the park a little. It's huge, but the area near The Prado is easily walkable.
Oh, if there's a baseball game, you can get tickets to the Park at the Park. It's the grassy, hilly part of Petco Park where you don't have to give a shit about baseball to have fun. Other recommendations depend on where you're staying and what you want to do.