We usually book while on a cruise for the next cruise. We put $100 deposit down. When you get home, you receive a "Future Carnival Vacation Certificate" that gives you an on board credit from $50 -$200 depending on the length of the cruise. You have 24 months to ake the cruise. You pick the cruise and they will apply the $100 deposit. So, you basically get an on board credit by booking a cruise while on a cruise.