Kasi1979- I am so sorry I did not see your post until now.
We have been to GC 3 times. The last time we booked our own excursions (online before the cruise) It VERY easy to do in GC because most of the tour operators (at least the ones you would find online) also contract themselves for the ship tours. So, they will all meet in the saem place AND since you will most likely be on a tour with ship tour people, you are almost gauranteed to get back in time because you are riding with folks who already have that insurance. You can a little money doing your own, so if you are comfortable with it, I say go for it. Many people on this site have used Captin Marvins...
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