My motto always was, "if there's time to lean, there's time to clean". Keeping these young employees busy during slow times is always a challenge. However, there is always something to clean in the concession stand... We always had an "in-between show checklist", as well as a "daily job list" for each shift. There were the typical sweep the floor, wipe the glass, etc, stock the concession stand, etc type things, then say every Monday, they wiped down all the formica in the vending stand, or totally dismanted and cleaned the butter machine, etc. Just give them any busy work you don't feel like doing. I've had people fold newsletters that were being sent out, stamp the return address on envelopes, decorate for holidays, promote for movies, design flyers, anything...
It really depends on the size of your operation how early you need people to come to work. A half an hour seems reasonable to have the stand stocked, cleaned, fresh popcorn popping, and other food items ready.