My husband received a package from Yahoo once, and when he opened the package, it played the yahoo yodel sound. I took the little sound maker from the package and put it in my drop box, so whenever anyone uses it, they get a 'yahoooOooooo.' The kids like to come by the office and lift the flap when they get home from school too.
Hopefully it at least gives residents a smile as they are turning over their hard-earned cash for rent!