Make a unique print by cutting up a potato or sponge, and use it to stamp on a material of your choice.
Disclaimer: I really wanted to work with a potato. Unfortunately, the grocery store near my house doesn't sell large potatoes by the unit, and those mini-potatoes would have been more trouble than they're worth. So i settled on option b, sponges.
It's only when i opened the sponge package and started removing the 'heavy duty scratch pad' part with an x-acto blade that i noticed the packaging saying that the cleaning agent was built-in, and to just add water! Unfortunately, this meant goop residue all over my hands, blade, ink pad, work area... let's just say that i couldn't be done soon enough. Maybe i should have used the second, uncut sponge to clean up?