Throwing a party for a teenager? Stage a scavenger hunt in your yard, your neighborhood, at the mall or at a local park. Create a list of items that must be found -- if you're playing in a park, you'll probably also need to hide items to be scavenged -- usually about a dozen or so is the sweet spot. Break the group into pairs or small teams, give them the list, a timeframe and anything they'll need to get the job done (such as a digital camera) and set them free.
This idea works for the young kids too, if you scale back the idea. Consider a search party or a game of detective around the backyard.