I asked the physio to give me some exercises for the BOSU (see above) as I feel my balance is still off. He gave me some things to do when it's turned right side up (as shown) but I found them heavy on my ankles and feet. So today I tried something different and turned it over. Standing on the hard bottom, the muscles to balance it are higher - stomach, lower back. It's more a core than a leg exercise that way. It's way harder, too.
It was also great fun, like standing on a dinghy and stepping over to another. Next time I'm putting a bunch of them in a row (along the wall) and will practise stepping along :-)