It also depends on the port. If we have been there before we may not bother with an excursion. Or, there are some ports where there isn't any excusrions we are interested in.
AND then there are the ports were we are comfortable enough in doing our own thing (no cruise line booked excursion). However, like Penny, its typically one excursion (be it ship booked or self done) per port.
It can also depend on your budget. If the cruise fair is about all your budget permits, then there is nothing to be ashamed of if you simply get off the ship and walk around and sight see or shop without booking an excursion. Most of the ports can be done like this as the local economy is based on the cruise ships and the local shops get placed very near to the cruise ship docks.
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, Freedom x 2 6/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013