We took a taxi to Sapphire Beach. The taxi on the way there was $10. The beach is nice. It was not very busy. Most ship excursions go to a different area, so this was not crowded. We rented snorkels for $5. There was quite a large coral area with fish and even a turtle.
On the way back to the ship, we took the local bus. The local bus is exactly like the taxi, but only $2 and it stops a bit more often. To take the local bus, you simply wave them down and hop in. At your stop, hop off and pay the driver your $2. The locals were very friendly and helped us figure where to catch the bus and when to get off the bus.
Barbados was an amazing port. The city is very nice. We did not bother taking a taxi. We walked to the beach, which was only about a 20 minute walk from the cruise terminal. Just grab a map from the terminal and set out! Don't let the local taxi drivers fool you by telling you it's a long walk. It is very short and pleasant. The beach is perfect. The sand is white and powdery and not hot.
Once we got to the beach, there were a few options. You could pay for chairs and umbrellas for around $10, or just bring your own towel and set up. There was also an area with a pier where there was a rope swing and water toys that looked fun. Beside the lobster restaurant you can find a nice boat tour for about $20 that will take you out snorkeling. We paid $25 each, they took us out to snorkel with the turtles and then to a shipwreck to snorkel with the fish. The boat drivers throw food out so that the turtles come very close and will even let you pet them. It was very pleasant.
I think if we had done the same activities through the shore excursion from Carnival it would have been about $100 each, so it was definitely a huge savings to do it on our own.
We chose an excursion on the "No Links" boat from the vendors at the cruise terminal. You take a fast speed boat to Soufriere, stopping along the way in Marigot bay and to see the pitons. We transferred to a van which went up into the volcano and to the waterfall. Then they took us back to the boat to a beach for about an hour of beach time, and then back to the port. It was really very fun. Our tour guide was very fun and personable, the other staff on the boat and van drivers were all friendly and helpful. The alcohol was flowing and everyone was having a very very good time by the time we returned to the ship.
DO NOT RENT A SCOOTER IF YOU ARE NOT AN EXPERIENCED SCOOTER DRIVER. We rented a scooter for about $80. This is not something I would recommend, unless you are experienced at driving a scooter. People will tell you that they are easy to drive. Well that may be very well true if you have an opportunity to learn to drive it in a parking lot or a subdivision at home. If you rent a scooter in St. Maarten, your first go at it is on a busy main street with aggressive drivers and a lot of honking. People are crossing the road everywhere, they are merging on the roundabouts without signalling, there is traffic and construction etc etc. On top of all that, it is a very uncomfortable ride for the person in the back. For only a few dollars more, you can get a vehicle that has four wheels. DO NOT RENT A SCOOTER IF YOU ARE NOT AN EXPERIENCED SCOOTER DRIVER.
That being said, if you rent a vehicle, it is very easy to get around the island. It is only about an hour to drive all the way around. Drive slowly when going downhill because there are some very sharp turns.
We spent some time on the beach watching planes come overhead. The sand is nice and the water was very pleasant and clear. We went to the french side which was lovely and stopped for lunch there (delicious). They took USD for lunch, but normally deal in Euros. The cafe we stopped at did not accept credit cards.