Stuffed Swiss cheese
Buy a real Swiss cheese with really big holes. These holes can be stuffed with bills. You should cover them with plastic foil or silver foil and stuff them into the holes that are easy to reach.
Another nice creative idea is to place some mice on the cheese. You can find models for those in books and in the internet. Fix them on the cheese with toothpicks and, if you want to, write a funny saying on them.