if you have an NXT kit, have some of them work in a group to build the basic design that the kit comes with. It takes all of 5 minutes but they will have fun with it and building a simple robot. If you have time, have them make a sort of very simple Rube Goldburg invention with some of the VEX or NXT parts. There doesn't need to be a specific task to accomplish for that but you could have their RG invention do something as simple as use 3-4 simple machines and as an end task, release a marble or something down a slide.
And remember: Kids love to get free little giveaways.
