Most kids are potty trained around age three. Around age three is also the time kids start getting an interest in numbers, in counting. Why not combine it? For my kids after that initial day where you take them every 20 minutes, I make them ladder charts. I start with charts that count to three. Each chart has 3 squares. Every time They go I place a sticker in a square. After three stickers they get a prize. I usually get dollar store prizes or make them friendship anklets. Some people use candy but i prefer not to. If my child needs a little more intensive then a sticker each time they go i'll give them a grape. (just make sure you don't give them grapes any other time) They can have other fruits for snack.
Once they start burning a whole through your pocket going three times in a row, make the ladder taller with more rungs. Five, seven, up to ten rungs.
Be sure to write the number in each square so they start to associate the oral number with the written form. They won't get it right away (unless you have a little genius) but it's good to provide them with lots of exposure so they can learn those pre-academic skills for before kindergarten.