...having strange mental image of a hamster in one of those acrylic balls rolling around all over the place... An armored rodent- the ultimate cat toy.
Some friends of mine used to have one of those for their critter. The little guy would put in some serious mileage around the house in it.
Scratch that idea.
I figure he'd just get stuck inside the first crater he rolled into and it would be just another footnote on the list of 'failed' mars missions.