Why Halloween is the balloon holiday? Easy to see: Because of the spiders and webb! Both a so easy to make out of balloons!
Spiders! 6 Legs, body and teeth! If you make it out of the balloons it comes natural. It can be a small thing, made out of two regular black balloons and 6 long balloons, or a huge one, suspended from the ceiling arches for legs. One important thing to make sure your Halloween spider has is teeth and a wide smile. Teeth the scary, smile... Well, how needs a spider on Halloween without a smile?
The webb can be black, silver or onage. As long as its color is opposite to the color of the wall behind it. Then you populate it with spiders, big and small. It there is a chance to use it in the dark, then balloon blinkers would be perfectly in place.
Good luck, have a great party!