Our 4 year old has been wanting her toenails painted for a long time. It's not something that I do much, nor do I want her to grow up too soon.
However, M got her toenails painted as an incentive for being dry three times in a row (2 naps, 1 nighttime).
Since then, we have seen many more dry naps and even nighttime pull ups. Yay! (I know many would find it strange that a 4 year old would not stay dry during a nap, but her naps tend to be several hours long. She does sometimes skip a nap, too.)
Next, if she gets a sticker for each of her toes (shown below), then she will get her own bottle of nail polish.