For the swimmers! Great promposal. This boy asked this girl to the prom by putting a sign in the pool during the last lap of the 500 meters at her swim meet. (note his dear friend behind him holding the flowers.) She said yes:-)
Love the idea of using balloons to ask/answer. It could be hidden inside one of the balloons, written on the outside of the balloons, etc. For this effect, use golf tees to stake balloons to the ground all over the yard!