I think it would be brilliant if Charles spent his whole reign pissing off the monarchists by doing things they find unsavoury, to the point where the monarchy is no longer tenable, but yet he and Queen Camilla have the most fun imaginable. Go out with a bang - knowingly destroying an outdated, not-fit-for-purpose institution whilst enjoying every last minute of the whole charade.
Name everything he can Queen Camilla - hospitals, boats, aeroplanes, gardens, palaces, etc. Even better, just as there are many examples of things being named Bowes-Lyon after HM The Queen Mother, name as many things as he can Shand.
Welcome Harry and Meghan warmly into the fold on their terms whenever he fancies, because it's no one's business how/when/where he sees his owns son and grand children.
Turn Buckingham Palace into a huge winter garden with plants filling it, hourly sprinklers raining down.
Stop toeing the party line with Andrew and aid his bringing to justice, whilst welcoming Beatrice and Eugenie into the family and making them working royals again.
Have a new National Anthem commissioned- one we can all enjoy.
Live in the tropics somewhere and refuse to spend their twilight years working their arses off, standing on barges giving themselves urine infections.
Start giving his free and open opinions about politics and politicians.
Sell off the rest of the palaces and give the money to charity, or turn them into accommodation for homeless people and asylum seekers.
Renounce the CofE and make it clear that he does not see himself as Head of any church.