Stickers are great rewards for students. Print-n-Play has fish stickers that can be placed inside a printable aquarium. It could be a contest for a month (or longer) to see how full each student can make their aquarium. This page from Steve McGuire has other great ideas to use including patterns for making houses, barns, and even space stations.
http://www.mcguirezone.com/goodies/toys.html
