My husband and I just sailed last month on the 16th - 23rd to the same itinerary. Here are the excursions we chose:
Cozumel - because both of us have been wanting to scuba dive but aren't certified, we picked Discover Scuba. It was roughly $110 per person if I remember correctly (which was the cheapest out of all the ports) and lasted about 3.5 hours. We loved it and it was the best part of the port as we were not fans of Cozumel AT ALL otherwise! They give you a safety lesson and get you right in the water, and you're in a great location with plenty of fish, coral, and statues to see. Hands down this was our favorite excursion of the cruise!
Grand Cayman - we ended up choosing Stingray City and Snorkel because I've heard nothing but great reviews. This one was around $60 per person. Unfortunately it was not exactly what I pictured, as all of the
DCL pictures lead you to believe this particular excursion will have you on a sandbar surrounded by stingrays - come to find out there are actually two Stingray Cities, and we ended up at the "original" (which is much deeper at around 13-15 feet with only a few stingrays). In order to get the excursion that has you in shallow water with the stingrays, you need to book the one that specifically lists "sandbar" in the title. Overall the excursion team was phenomenal and very hands-on, we got to pet a stingray, and had plenty of snorkel time off of the boat. We were content with the price we paid for the experience, but it was nothing special for us. (I imagine this would be a much bigger hit for kids though.)
Jamaica - we bit the bullet and decided on Bobsledding and Dunn's River Falls and we're both very glad we did! It's one of the pricier excursions at $140-160 a person, however it's a great all-day experience that gives you a little bit of everything. You're given an hour long tour to Mystic Mountain (our tour guide was awesome), then you hop onto a ski lift that takes you up to the top for the bobsled ride and complementary lunch. The views are stellar, and with luck, you will see some other cruise ships at Montego Bay on your way up. Lunch was delicious, and the bobsled ride (while short) was a blast and a HUGE hit with my husband (who is a big kid at heart!) Dunn's River Falls was a very fun experience as well and a must-see-and-do while in Jamaica as it's just so unique. Be prepared to get soaked!
Have a blast on your cruise!