You wouldn't even have to do that.
Just make donations tax credits.
If you Owe $10000 in taxes but paid $2000 to amatuer sport (or arts or whatever) then your tax bill would be reduced to $8000.
If you donated the full amount of your taxes owing then in essence you'd be telling the government 'I choose to spend my taxes here, here, and here.'
It'd pose a logistical nightmare to governments addicted to spending and never happen, but what fun we'd have.
