My gym has got these 'bosu' things which are wicked for balance and core strength. I just try and do any normal stuff I would do in the gym (less weight though), ie squats, lunges, shoulder exercises, various stuff with cables whilst standing on a bosu at the same time. I also like seeing how far i can jump onto it from and then maybe spin off. I don't like work for bosu or anything, I just find it makes the gym way less boring!