One room in a location was a quite high, shoe box shaped room, dreadful acoustics. 500 helium balloons, and it turned out really nice! Became the best room on that location. Our sound team were ready for some really bad jokes if it would have backfired;)
The balloons arrived pre-filled and with some coating that makes them last for a week. We saved three clusters so we could move them easily to other rooms as well, did good in the corners. And they were.....cute.