Well if you book a hotel in Cocoa Beach (near the port) you could do the NASA tour. They also have a decent public beach there.
If that's not your thing, there is plenty to do in Orlando. Besides Disney, Sea World, Universal Studios et al... you can also go to City Walk area near Universal Studios... you'll find The Hard Rock cafe, Planet Hollywood (I think), you can see the Blue man group there, there is a Margaritaville, NBA City.. etc... Or you could go hang out at Disney's equivalent - Downtown Disney....
If you can't find something to do in Orlando for a day/night, you didn't put any kind of effort into it.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise, Magic 1/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013