My husband and I did excursions in both Nassau and Castaway Cay. Now I was dissapointed with Nassau I wish we'd just of stayed on the ship and enjoyed the ammenities there. We did enjoy our excursion, we did a harbour tour where they showed you where all the celebrities lived on Paradise Island it was nice. We then explored the straw market and took a cab over to Atlantis. Atlantis wasn't that great to me but that is just my opinion. We were there one of the nights that the ship is in Nassau til 2am. I think I'd op to stay on the ship next time for Nassau and ride the Aqua Duck, maybe do a spa treatment etc.
For Castaway Cay we did the glass bottom boat and the stingray adventure. I still talk about swimming with the Stingrays it was so amazing! Id do it everyday it was so much fun. The Glass Bottom Boat was nice too I'd even do it again now that I have a son (I had my son after the cruise actually he was my cruise souvenir LOL).
Id definetley spend more time on Castaway Cay and maybe just wander around Nassau yourself see if you like it and go from there. Oh on Nassau lots of people will approach you selling stuff such as tours, hairwraps etc.