>>What do you do to optimize your balance?

Balance board at the standing desk.

About five years ago, I was in hospital (SICU) for about a month (brain infection). had to re-learn things like walking and getting up off the floor - and my balance took a hit (used to be good enough that I could walk on a railroad track and read a book at the same time) so I've been focused on balance. I've been using the length of time I can stand on that board on one foot with my eyes shut as a way to judge progress.

+1 on Tai Chi. For anyone looking for an easy way into it, check out Yongxin Li's 'Easy Tai Chi'. For me, it's been nearly as useful for balance and flexibility as aikido - without getting put on the floor (Aikido doesn't 'throw' people, but still...)

And, speaking of aikido, I'd take issue with your dis-recommending 'barefoot', as that's the absolute best way for sure footing in the dojo. Sock-foot comes second, but then slippery floors are a hazard.

+1 on swimming, but, in my area, outdoor water is stiff... and indoor pools are problematic. Can't wait to get back in the water.

Also, are you recommending vitamin D for bone health or balance? If it's the former, why not K2 as well? My understanding is that D alone helps calcium absorption but does not direct it to the bones.