While I've never stayed on the ship the entire time we were in port, I have gotten off for only a short period of time.
I don't think it's weird at all and many people stay on the ship in certain ports. There is plenty to do, although not as much on port days. There are, however, often very good deals in the spa on port days.
My favorite days are days at sea and have been since my first cruise. Enjoy!
CarnivalSensation February 2015
Carnival Dream November 2015
Carnival Fascination April 2014
Carnival Elation March 2011
Carnival Imagination Sept 2007
Carnival Sensation Dec 2006
RCI Sovereign of the Seas Sept 2006
Carnival Miracle Sept 2005
Carnival Glory Sept 2004
Carnival Fantasy Jan 2004