We only took two of the shore excursions offered by Carnival - the Jolly Roger Party Tour on Barbados and the City/Barcardi Distillery Tour on San Juan. I would recommend both of these tours. The Jolly Roger was alot of fun and our whole group (14) went on it. I would not recommend taking kids on it however, since it got pretty wild. The City/Barcardi Distillery Tour was a good way to be the first off the ship (no sitting around waiting for your color luggage tag to be called for disembarkation). We got a great tour of Old San Juan, did some shopping, ate lunch and then toured the Distillery. (Very interesting & great samples!) Our taxi driver got us to the airport about 3:30 and our flight was at 4:00. Perfect timing!
My sister and her husband did most of the snorkeling tours. The one they most talked about was the Champagne Snorkel on Dominicia. She said this was the best snorkeling experience they had ever had. Two of the other couples did the SomeTing Cool excursion on either St Maarten or Barbados (I can't remember) and they said it was a waste of money. We hired a taxi in St Maarten to take us to the beach. $5/pp each way. He took us to Orient Beach - the one with the clothing optional side. We rented a chair, umbrella, rum punch and a ticket for the bathroom for $6/pp.( Each addtional trip to the bathroom was .50) The water was unbelievably beautiful! We also hired a taxi on Martinique for $20/pp and got a tour of the island. He took us as far north as St Pierre, through the rain forest, then down along the coast. It was a beautiful drive and we all felt we got our money's worth. It took up a better part of the day. We even toured a banana plantation. He dropped us off in town for shopping and we walked back to the ship. I think if you see a shore excursion that sounds like you'd enjoy it - go for it. If you're just going to the beach or shopping you'd probably do better on your own. We found the Cruise Director to be very honest about the excursions. Ask him.