Impromptu game: "Clean Your Room!" I throw a bunch of foam balls and pool noodles into the center of the gym. The kids are split into two teams - each guarding ('cleaning') one half of the gymnasium. They have a predetermined length of time to throw the items as far in to the other half of the gym as possible. The side with the least amount of stuff in their room when it's over - WINS!
*A substitute was found within two hours and classes resumed as normal for the rest of the day.