About the only untaxed item left (which is apparently under evaluation by the present 'lot') is breathing / air.
The proposal :
On arrival, you will be given a lung capacity (LC) evaluation and a respiritory rate ) per minute measurement, (RRM) this will then be multiplied to give the volume of air consumed (LC x RRM= W ) and then W multiplied by the duration of your stay (X) and the 'agreed price per litre' (Y) and taxed at 20% of that value, giving a figure (Z) that will need to be paid.
It will be paid in advance by an "average" security deposit when applying for a visa, and any credit refunded on your departure. If you have been undertaking any strenuous excercises** and you have exceeded the value of your security deposit you will be held in cells at the airport / port of departure until you (or a friend / family member) can clear your account.
** Any activities where you can provide receipts showing the 'paid for' use of 'air' such as High altitude flight, Scuba diving, or surgery in hospital, will be credited back to your account.