For the best return on your vacation investment, consider buying a Future Cruise Credit while on your family cruise. For a modest investment, you are guaranteed the value of your investment applied to the deposit of your next cruise AND an additional on board credit.
Carnival Cruise Line offers Future Cruise Vacation Vouchers for $100 during your family cruise. Invest $100 into a FCV, use the invested $100 toward the deposit of your next cruise and depending on the length of the cruise and the cabin type, receive additional on board cabin credit - minimum of $25. Not a bad investment return.
Princess Cruises offers Future Cruise Deposits to their guests on board. For a $100 per person deposit you can secure your next Princess Cruise and collect on board credit for each passenger for all 7+ day cruises. Inside cabins get $25 pp and outside cabins $50 per person on board credit. That is a 50% return on your investment for an outside cabin on your next 7 day Princess Cruise.
Of course these are just examples and there is some "fine print" at the bottom of these certificates. Most certificates are good for at least a year from date of purchase and are combinable with other promotions. Restrictions vary by cruise line so remember to ask questions.