Princess Gift Cards, $500 will get you $550 total
Time again for the Princess Girt Card 'sale' - if you buy a $500 card you'll get another $50 card for 'free' . While it indicates that there is a limit of one, I purchased 5x$500 cards and got 5x$50 extra gift cards. https://www.princess.com/learn/cruise-gifts-celebrations/overview/gift-cards/
Based on their FAQs excursion payments are not allowed:
“The Princess Cruises Gift Card can be used for most Princess products and services, with the exception of payment towards Land Excursions and eZAir”.