I think an upgrade path is a good idea.
Hey, NnB, since these things come in pairs, maybe one could have a motor and battery in it, and be linked to the other by a short flexible casing -- a smaller, lighter duty, stainless steel version of bicycle brake cable casing -- containing a flexible driveshaft. Inside the other ball, there could be an eccentric cylinder on the shaft. The spinning of the eccentric mass would case both balls to dance around on the ends of the flexible line.