I punched out 2 inch circles in cardstock that say "I've read 20 minutes", and "I've read a book!". I distressed the edges using Distress Ink.
I thought the easiest way to keep track of all the circles was to use one of my favorite things...buckets! I already had them for each of the kids, and they're super easy to keep out for long periods of time. Ours go on a ledge in the family room, where they are in plain sight, and are easy to add to.