What you describe is the polar opposite of freedom. It should be immediately dismissed not due to the current political climate but instead because it would establish the ultimate nanny state full of red tape and bureaucracy.
No one should be arbitrating personal activities unless they directly and immediately impact those around you.
(And for the record, I vehemently disagree with sin taxes such as those for alcohol and tobacco. Legally requiring health warnings on the packaging, on the other hand, makes a lot of sense to me. Also note that alcohol and tobacco have much more consistent and severe health impacts than, say, refined sugar.)
A nonpartisan government panel of scientists. I realize with the current political climate in the us, that might be a tall order, but that doesn't immediately mean the idea should be dismissed.
>How will they go about determining whether there are more benefits or costs associated with a particular activity?
Cost benefit analysis based on empirical evidence?
>People are already incentivized by nature to sort out on their own what sorts of activities will allow them and their communities to flourish. Let them. We will all be better for it.
The examples you've listed lie on a spectrum. Indoor rock climbing probably isn't too negative, all things considered. OTOH motorcycles is much harder to defend. You're really going to risk your life to save a few grand a year on gas and parking?
Like I said before none of this is new. The government has already deemed tobacco to be more harmful than good. As such, it's heavily taxed. I don't find it outrageous to tax things that are similarly obviously more harmful than good.