We used chocolate buttons for wees - one wee for one button, but like Heaven said we soon needed to stop that because DD2 could squeeze out seemingly 1000 little wees a day. We ended up not needing any reward for wees, but for poos they got a sticker for each poo in the toilet and once they had X number of stickers they could choose any toy they wanted. DD1 got toys and DD2 3 toys, then there was no looking back.