Back when I flew out of SJC a lot, I'd get a sandwich at the in-terminal Max's.
Back before the terminals got redone at SFO, and pre-9/11 security enhancements, I'd usually check in, then go through United security to get food there, then out of security and to my flight on the AA concourse. That ended with the post-9/11, until they redid security to have a single shared zone.

These days, my favorite carry-on fast food meal is the bacon-ranch salad from McDonalds if I'm going through ORD or DFW where I can get one. At SFO, Boudin and Subways both do a decent takeaway sandwich (Boudin's better but not always worth the walk)...

I also generally bring a snack bag of my own - some dried fruit, some beef jerky and/or slim jims and/or salami, some individual package cheese, and a bag of crackers. Sometimes also some sliced cucumber or an apple, although those don't keep as well if I don't eat them on the originally-planned flight. The other must is to buy a big bottle of water, airside.