Buy 100 shares of stock, get OBC to spend onboard. Buy a FCC once onboard for your next cruise, for more free OBC to spend onboard.
I am already at $200 for my solo 7 day cruises before I have to pay anything. My tips are paid for and at least one excursion, depending on how long the cruise is. The above are combinable. For a 5 day, I would have $100 to spend.
I don't really spend much over that, as I already have bought so much they sell in the stores and get a lot of free drinks, (especially on RCL), but Carnival has two events with free drinks right off the bat.
Other than tips, you can spend nothing, or a lot. Tips are the only mandatory spending I feel.