Hmmmm... I am not sure how I feel with the idea of rewarding a child that age for something so simple they should be doing anyway??? I know our DDs are the same age. They are old enough to reason with. Or, at least, we should be able to reason with them!! If she does it again you could leave the water in there until the next night. Then, at bath time innocently say, "ok, hop in". She will be aghast. Then say, "oh sorry, I thought you must have been saving the water from last night to use tonight!! I cant imagine any other reason why you would not pull out the plug...'

Obviously, don't make her bathe in the water! But it might make her think....

Either that, or go for the shower idea