1. Divide players into two teams and take turns batting and fielding as usual. Rather than calling strikes, you may want to change batters after a certain number of swings, and switch places whenever all the players on the at-bat team have had a turn at the plate.
2. Give each team a point for every balloon they break.
3. The outfielders won't have much to field, but they can try to tag the runners before they get to base.