One thing I saw that was selling like hotcakes was a balloon "yo yo". They had taken balloons and filled them half full of water, painted some swirly-Qs on them with paint pens, and attached a rubber band to the knot. Kids could put the band on their finger and bounce them up and down. The kids LOVED them and hardly a one passed the booth without buying one. I think they sold for $3, but everyone selling them sold out!
Smart idea...