S.J.
I used cotton balls to reach a goal. I had a tall plastic container that his growth chart came in and my son and I put stickers on it together and used blue paint tape to go all around the container about 3 inches from the top so he can see when it goes over.
Every time he sat on the potty, he got 2 cotton balls in the container. Pee pee or poo poo got him 4 cotton balls in the container. Once the cotton balls went above the blue paint tape, we go to the store and he gets a big toy. I would give him the cotton balls to put in himself.
I chose the cotton balls as they were big, cheap and reusable. Plus no sugar, like M&Ms. And we got to practice his counting. He liked the visual and was proud to show his grandparents when they came over.
Good luck!!!