See those snazzy Cat in the Hat hats? We stuck some up all around the children's section of the library and encouraged kiddos to try to find and count them all (I put up eleven in total). Every child who reported the number of hats he or she found, regardless of the number reported, got to choose a Dr. Seuss-themed reading sticker and a Read Across America bookmark. It was simple, it was cheap, and it impacted a lot of library customers.
This seek-and-find was a great opportunity to mix simple math skills into a passive library program--at least twice a day I heard hat-related counting going on throughout the building. Kiddos got really excited at the prospect of a treasure hunt in the library, too, and they loved that the theme was something as recognizable as Dr. Seuss.
Give a seek-and-find a try at your library! I'd love to hear what themes and activities work for you.