Build a device that moves Lego soccer/basket balls from and to standard-size hoppers, then bring it to the Convention. Everyone’s devices and hoppers will be lined up to form one great contraption.
The building standard and some beautiful examples can be found at the Great Ball Contraption Web site.
Some works in progress can be viewed here, here, and here.
If you plan to bring a particularly large module or if you have any questions, contact theme coordinator Kevin Mitcham at kmitcham@gmail.com.
If you don’t have a spare motor, consider building a module that will accept power from a shared drive shaft.
Here is how it works:
A technic brick two bricks up, one stud in from the edge on a 16x32 baseplate, with the hopper on the other edge. Roughly 100 rpm clockwise (full power on a Power Functions Medium motor) , as viewed in the direction of ball flow. The axle should be flush with the edge of the baseplate.
http://www.flickr.com/photos/kpram/3711638450/ is a link to a photo of what the coupling could look like.