Before the Party:
Purchase small plastic Army men in at least two different colors (most readily available in green and tan). Hide them throughout the play area.
During the Party:
Break the guests into two teams, assigning each team a color. Send the guests off to hunt down the men, with the understanding that they will receive one point for finding their own men, and two points for capturing the opposite teams' men. Whichever team has the most points after all the men are found, wins!