Talking some inspiration from this, here is a variation that you might try for a fun challenge with your NXT set and any other LEGO parts you have to add to it: Move the larger NXT balls around, and for an extra challenge, make the motion continuous so that the balls each travel in a continuous path in, around, and/or through and back to the start of your machine without any human help. Never mind about modules and standards for multiple builders for now, just see how interesting of a way you can come up with to keep the NXT balls in continuous motion.
Here is an example NXT Ball Roller Coaster with building instructions for a single NXT retail kit that does this:
Obviously the more additional parts you have the more interesting it could get. I would love to see somebody do something cool with flexible pieces if you have some of them, for example. The possibilities are endless, really. And you don't need to be limited to a track type idea either, think of other ways to keep balls in motion. For example, this Mystery Machine keeps one NXT ball in motion in kind of a silly way, and there are lots of possibilities involving throwing/dropping/etc., as long as you can catch the balls and keep them going continuously. If you know of a link to somethink cool that does this or build something of your own, feel free to comment with a link!