7th Inning Stretch

Thousands of huge balloons are released into the air by one team's fans at the top of the 7th inning, then by the other team's fans in the middle of the 7th. The balloons have some type of combo noisemaker/propulsion system so it's noisy and the fly around.

If you see a few errant balloons, it was probably just dorks like us who knew we needed to let the balloons go with everyone else, but let ours go with the wrong team. The row of Japanese salarymen behind us found it hysterical.