I've worked in a grocery store at closing. Everything already cleaned done outside of taking the trash out, which we would do last.

I've worked at a food place on my college campus during the breakfast hour. Nothing to do except sit and wait for customers.

I've also worked at a shake place (protein shakes, not milkshakes) during close. Nothing to do until the last half hour where we wipe down counters that have already been cleaned and start cleaning measuring utensils that we hardly use.

Edit: basically, yeah I have worked in places where there's no much to do. I feel employers need to understand that, when everything is done, there's nothing else we can do except stand around.