Buy medium sized water balloons and fill enough for each guest to have multiples. Make all the guests stand at a start line and give each one a water balloon. Have the player hold it between the knees and race to the finish line without dropping it.
If the balloon falls and breaks the guest has to shout, “My water broke”, run back to the start line, grab another water balloon and start over. The guest who reaches the finish line first with their balloon intact wins.
For a variation you can also hand out prizes for the person who breaks the least (or most!) balloons before the end of the game. Be prepared for lots of water on the floor and this game is best played outdoors!