Split the kids into teams of 4.

Before the party, make up a list of items inside or outside the house that the teams have to collect. Then give them a limited amount of time (depending on how many items you have asked for and their location) to find them.

When the time is up, call the teams back in.

Whichever team has the most items from the list wins. Alternatively, each item could be worth a certain amount of points and you total it up that way.