-one of my lifesavers - SLIM JIMS - fit well in the back pocket or in a purse
-small bags of peanuts
If you are going to be there for awhile, most people have a can of tuna laying around - while your friend is eating a sandwich, see if it would be OK to fix some tuna & mayo to munch on. Or, carry some of those little packets of tuna in your purse/bookbag.
Your friends may think you are a little off, but once they see the results, they will understand