Wheels are bad for them, so are leads and harnesses and I'm not aloud to let them run around in a room which I know would be really fun for them. So how else can I exercise them? Obviously I can't let them run around the yard because there are cats everywhere, and a million places where they could get out. Are there any other ways I can exercise them? They go in their playpen nearly everyday, but they don't run around in it, do you have any ideas?