Snacks & drinks are totally fine. We're usually a large group so we bring in lots of edible goodies and drinks to help us get through the day. We've asked at the security check on more than one trip if what we had with us was ok, and we've always been told the same thing: yes, and no glass & no alcohol.
I've mentioned this before, and it's totally true, I once saw a lady try to bring in a bucket of KFC chicken, and it was denied. I've heard things about pre-made deli sandwiches, but it was more like 4th-hand information so I don't know about that or fast food, though we do bring in value menu chicken sandwiches in.
Things we usually have with us:
Several cans of soda
2-3 Powerade bottles
chips, cookies, pretzels
jerky, energy bars, nuts
bread rolls, candy, and a few hamburgers & chicken sandwiches concealed here and there.
I've never tried bringing in a mini 6-pack size cooler, I have this small neat cooler lunch carrier which I think I'll bring with me on my next trip.