Drape a piece of tulle over the balloon and gather it at the bottom. Attach flowers (silk, dried or real! If you wrap the bottom with cheap hair elastics you can tuck the stems into the elastic band, hiding the band and adding a splash of color.
If I had a pool I would totally try this out. Put a glow stick in a balloon for pool lanterns. Pool party on a Summer night! I think this could work pinned up on the fence of a backyard without a pool, too, so really great idea for any outdoor BBQ/party!