I would highly recommend Trader Joes Force Primeval Energy Bars. I stick them in my jersey pockets. Good for energy and keep you full, and lower sugar than standard energy bars.
Regarding your lock......I ALWAYS lock up my bike. If you are fine with it being stolen then don't worry about locking it up. You're rolling the dice each time you don't lock it up, and for me the gamble is not worth it.
If you are just doing a few minutes in a store, you could use just a light weight cable.