They could float around whatever area they were placed in, and move when someone runs or jumps into them, like the Plexus ball. Except, if you hit the balloon too many times, it could pop!
My request would be to be able to not only have balloons, but be able to tie a note to them and turn them loose. They would fly up out of the street and then after some period of time they would drift back down into some other street somewhere and fall to ground level where the note could be picked up, read and detached. deflated balloons could be donated for 1xp, much like notes.
I like this, especially WalruZ's additional ideas!
Also--we could use them like fortune cookies, pop and get a note or small prize?
Or--let us put cubimals in them. Can you imagine CUBIMALS IN BALLOONS? It'd be a great way to use up the 10 million chick and pig cubimals I usually just leave in trees for noobs.
Allows you to float any item to a non-underground street (and if it's an animal, not overpopulated) with a pre-written (but modifiable) note saying where it came from and who let it go? Fun!