Usually something small and quick is good.
I agree with taking an apple or even a banana.
Nuts and eggs are a good choice too. Both have a good amount of protien to help your muscles recover after working out. You can eat a boiled egg or two (keep in a small tupper-wear container), or cook some eggs when you get home.
The string cheese idea is good too.
And I would highly suggest drinking some vitamin water or juice to give you energy and help fill your tummy.
(And try to stay away from the high sugar "health" energy drinks - double check the nurtrtion info on the back).
Have fun and good luck.