Other than tips and drinks, you really don't HAVE to spend any money on the ship. If you want to spend money, there is plenty to spend it on. Excursion, cabs, watersports, and shopping are a few things than we spend money on. I think your question was pertaining to hidden cost. The tip for your waiters and cabin stewart are the only thing I can think of that's not mentioned, but expected. Roughly $150 will cover that, which is paid at the end of your cruise. Chuck listed some special services that would cost you $$, but it's your choice. Go for it! You only live once.