I've seen endless incarnations of this project in teaching books and on the web, but I finally sat down and made a couple to go with our pond theme.
I'm almost embarrassed to show my "artwork" to anyone, since it looks like the kids made 'em, but I will.
Turtle stick puzzle.
These are quick to make, and a little more challenging to put together than they look. They require only popsicle sticks, permanent markers, and tape to hold the sicks in place while you're drawing on them. They're an easy way to add a hands on element to whatever you're learning about.