My 4 year old loves balloons. This looks like a fun activity for those days it is to hot to go outside. Another great idea is tying balloons on strings and hanging them from the ceiling for them to hit. We did this in our garage.
Now this is a great idea... cardboard box birthday party for toddlers. i love to see childrens birthday parties made for the kids to enjoy, not just for their event planning mamas. I admit I was one of those Moms for a little while. Live and learn.