Split Jenga pieces equally and sit in a large circle (on the floor or at a big table). Everyone has one piece that is marked. Then everyone builds a fortress to protect their marked piece (the marked piece must be stood up on its small side).
Everyone gets a projectile. We liked using those thick rubber bands that came on broccoli at the grocery store, but you could use Nerf guns, balled up paper, whatever--as long as it's not too light and not too heavy.
Once everyone is ready, take turns firing at each other's fortresses, trying to knock down their marked pieces. Nobody can touch their fortress at this point.
From my experience, kids will always find ways to create and use imagination as long as they get the opportunity. It seems more difficult today than when I was growing up, just for them to find places they are allowed to congregate.
And my 3 year old has a very deep appreciation for a good stick.