Where can I buy a nice picnic lunch to go in San Diego?
I guess it's not really a picnic lunch, but we used to get a "sack lunch" from The Little Fish Shop in Mission Valley (near Ikea). A cup of chowder, bread and butter, and a piece of smoked fish and we would jaunt down to Balboa park and eat in the sunny spots. It's not a menu item, but I've never had them tell me they can't do it.
If it's just for a couple of people, Bread and Cie does a nice "lunchbox" packed with a sandwich, a slice of lemon cake, and some chips or grapes. You can also get good gourmet sandwiches at Con Pane, in Point Loma - but call ahead because they can get backed up when they are busy. I recommend the roast beef with goat cheese and red onion!
Saffron chicken, on India Street at the foot of Washington, also does a good takeout picnic, with roast chicken, jasmine rice and cucumber salad. You get a choice of sauce - peanut, sweet, spicy, etc.
Waters' Catering runs the cafe in the sculpture garden at Balboa Park which has good sandwiches you can pick up to go and take wherever you want to go in the park. You can also eat on the grass in the Sculpture Garden, which is kind of fun (and they allow dogs.) I like their chicken salad sandwich.
If you are looking to pick up an assorted bread, cheese and snacks type picnic - Whole Foods is a good choice - it's on University not far from the park. If you live near UTC or North of there, you might try Bristol Farms - or there's another Whole Foods near there too.