On Princess Cruises, you can still charge Casino chips to your room (don't think any other cruise line lets you) which you can pay for with your miles earning credit card at the end. If all you wanted was the miles you can max out the charge, cash them in, and use the money to pay off the card.

there is a daily limit and a per cruise limit per cabin, think it's $500 per day and $1,500 per cruise.