Gather a bunch of flat rocks
They need to be flat for checkers so you can stack the rocks on top for situations where a piece gets "kinged."
For checkers, you need about 40 rocks. For tic-tac-toe, you need about 12 rocks.
You can avoid painting the rocks by choosing two different colors of rocks. Or you could play tic-tac-toe with pine cones or other objects instead of rocks. But if your rocks are many colors, you need to paint them.
Wash the rocks before you paint them to ensure the paint sticks.