Doctor's Cave Beach- Jamaica $5 for taxi per person and $5 to access the private beach, then $5 each for umbrella or chair. We rented an umbrella and sat on our towels under it. It was a beautiful beach. They have 2 water trampolines and you can sign up to parasail for $55 per person. You can see the cruise ship from the beach.
Captain Marvin's - Grand Cayman. They have a website and I have used them for both times we cruised. They have a stingray/snorkel excursion and it costs $35 per adult and $27 per child. The boats aren't crowded and overall they are great!
Chakanaab National Park- Cozumel. We did the Dolphin Discovery Dolphin Encounter with them at about $100 per person, but if you sign up thru their website you get free admission to the park and free transportation to the park from the cruise pier. It was amazing! And much cheaper than through the boat. Just be aware that you can't book any excursion after 1:30 their local time otherwise it will put you cutting it close getting back to the ship due to the time difference on the ship vs their local time. There was someone that did this excursion with us that said they have done the dolphins in all 3 ports of call and this one by far was the best and most thorough of them all.
Happy cruising!