Carnival hosts a "party" for past guests one nite while on a cruise, lasts about one hour. Party is by special invitation. We have been on six Carnival cruises and that is ALL that we have ever received.
My wife is a TA...and she says that's all they offer is a party. If you receive Currents Magazine (one Carnival sends out) there are coupons inside that you may be able to use against a cruise (if you have not yet deposited the booking). After our first cruise (1990) we got the magazine and over the years gotten post cards for good deals from them. Upgrades are normally available on certain sailings and on certain ships, so your lucky to get one if you pick the right ship and date. As a past guest you would have been assigned a Past Passenger number. If you go to Carnival's web site you can get their 1-800 number and ask for Guest Relations and they can search for your past guest number if you cannot find it. This number would need to be in the booking prior to sailing to be invited to any parties. Hope this helps!