My favorite books for that down-time/extra two to three minutes before lunch or between classes are the Greg Tang books. My students love them. I first came across Grapes of Math at Wal-mart! It was a Scholastic book and it looked nifty so I picked up a copy. I LOVE it! Grapes of Math shows the students pictures of objects along with a riddle. The objects are grouped so that they are easy to count quickly. The goal is for the students to not count one by one but to find the pattern or grouping situation. I like to put my book under my document camera. There are several Greg Tang books that follow this same concept. Math Potatoes, Math-terpieces: The Art of Problem-Solving, and Math for All Seasons: Mind-stretching Math Riddles are a few of his others.