Genaille-Lucas Rulers are pretty cool. They are a clever calculator that you can use to multiply some pretty big numbers.
MathHappens summer staff member Anh Nguyen made a colorful set from scratch in Corel Draw that you can download and print on a laser or inkjet printer. Feel free to copy, explore, and use. It’s a great project to try to reverse engineer and figure out how they work. (You can also check the wikipedia for some hints)