The tender process is very slow with Carnival, so make sure to include plenty of extra time if you need to be on the island early. We did not book an excursion with Carnival because in my past experiences they have been overpriced and cheesy. Instead, we walked around the shopping area for a while and then found Island Tours offering the same thing Carnival did, but at $20 a person. We went to the Turtle Farm, which is a definite must-see. It was great! You get to hold turtles and the any kid would love it. Then we went to the Tortuga Rum and Rumcake factory. That was good, but you only need about 15 minutes there. They give free rum samples, which are about 1/4 of a shot, but are still nice. The rumcake is excellent. From there we went to the tourist trap Hell. It is basically a bunch of rocks with a wooden devil sticking out of it. As with the rum factory, you don't need to spend more than 15 minutes there, but it's kindof cool to see. Finally we went to the beach, which was great! It was beautiful and the ocean was warm. Anyone will enjoy it.
The best advice is to not book an excursion through your ship. Walk past the throngs of people attempting to get you to go on their excursion as soon as you get off the boat. Island Tour guides are everywhere. Agree to pay $20 per person and you will be all set. The Turtle Farm is an extra $10, and well worth it. Including tip your excursion will be about $40 total, per person.