I have not been to either of those ports in a very long time but I am pretty confident in saying that the shopping will be better in Nassau. I'll leave the excursion discussions to those that have been there recently.
Money Pits on the ship- Ignoring the casino, the big two are the Photo shop and the Spa. I find that the Photo's are the biggest money pit as you know going in if you are going to be using the Spa you can spend money there. The photo's can sneak up on your pocket book. They are constantly snapping photo's of you in hopes you will buy them. I think they charge $12 to $15 a piece (my wife does the photo buying so don't quote me). If I don't really try to control the number she buys we can easily spend $300 on photos. But I don't have too much to talk about since I send more than that in the casino..
The other big expense thing would be the spa. If you plan on having a massage or your nails/hair done or something like that you can expect to spend about $150 to $200 per massage treatment.
Shoes- Pick your outfits wisely so that you can use the same pair or two of dress shoes with more than one outfit. Then a pair of walking shoes or athletic shoes for sightseeing. So yes, 3 pairs is doable.
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