Here's a good website for comparing the fares and times of various airlines (except Southwest, which doesn't link to this): kayak.com

I agree with everyone that you should skip Tijuana and that San Francisco is worth a few days. It's not all charm, in that some downtown areas do have some homeless and there are gritty neighborhoods. And there's the chance you'll get some fog. But all in all, it's a wonderful, diverse, friendly town that's great for exploring on foot and that often has beautiful weather (if cooler than LA).

As others have noted, it makes more sense to fly in and out of SF airport rather than Oakland across the Bay, since BART connects directly to SF.

If you can only do a day trip, I think it's worth it. But more time in SF is much better.