I would recommend slack-lining as well if you are looking to improve balance. I also think it is safer than skateboarding too in terms of joint health and all of that.
I am looking to increase the impact, not reduce it. I know that I need more bone-rattling in my life and can't justify the purchase of a Juvent. Skateboarding provides benefits similar to that of a Juvent, plus I might learn how to fall (and skate).