I wait more like 45 min to an hour, some take longer to fall asleep. Then I give the awake ones a shoe-box sized tub of different activities. I just tell them that I have things to finish up, if they keep bugging me, although that doesn't really work. I don't know what ages you have but I have 2 prek girls who love puzzles so I also offer them a 48 pc. puzzle each that they can do. I offer the books you use with erasable markers so they can practice their letters. I offer folder games, there's 1 with a magnifying glass too. I went to the Family Dollar store and bought little odds and ends just for the boxes. Unfortunately the novelty wore off pretty quickly.
But they always love drawing and by the time nap time is over I have about 15 new pictures to post on the fridge.
Yes, it is a challenge to keep them quiet so the little ones can rest.