A great resource for games is the Cub Scout Leader How To Book. It not only has a tremendous number of games, but it organizes them, so you can quickly find a quiet game or an active one -- or, when your boys have finished early, and you still have 20 minutes: a game that doesn't require equipment!
I almost always include a game at the end of my den meetings; the boys have come to expect it, and if they begin to lose interest in the main activity, you can remind them about the game that they'll be playing later in the meeting.
