I was thinking of having a small contest each day where dragon balls are scattered around the convention space, and whoever brings them all to the gathering at the end of the day gets a great prize.
The dragon balls would be earth sized, as in the size of a softball, and marked with a bright flag. This would work the same way as the battle bucket used last year, except there would be 7 balls running around being dueled over. The duel is rock-paper-scissors, loser can't challenge again for 10 minutes, and only one dragon ball capture per battle. In the event of a tie, the person who has the most dragon balls gets the grand prize, while the others get a smaller but still worthy prize. In the event that 7 people bring one dragon ball each, they will all end up with the participation prize, and the grand prize winner will be up to the Eternal Dragon himself.