S.J.
For reward, 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. We did this at 18 months and talked about what we were going to start doing soon at 20 months.
At 20 months, 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!!!