We've done the Extreme Zipline offered by Carnival at Mahogany Bay. You get outfitted for your ziplining right there in the port, then RIDE up the mountain to start the zipline. The zipline ends right there on the free Carnival private beach at the port & shops. Super convenient and easy both ways.
We've only ever ziplined one other time (Belize), but the Mahogany Bay zipline was better in all metrics ... higher, faster, longer zips, longer overall, easier (not lots of hiking up/climbing other than the steps for the platforms), and more convenient as everything is right there at the port. We would 100% do the Mahogany Bay zipline again.
Note: the Carnival website has the excursion activity level as difficult, however we did not find it difficult at all (50's) and neither did my father (early 70's). According to the reviews, do not buy the pictures to be emailed, lots of folks reporting they never received. We took pics and videos ourselves, especially once the first person zipped into the bar/building at the end of the last zip. Just hang out there and take pics/videos as the rest of your party zips in (zoomed in or zoom lens recommended). ALSO: we bought snorkel equipment at Costco, walked back to the ship once we were done zipping, ate free lunch on the ship, got our equipment and snorkeled at that same free beach. There's a pier at the end of the beach that you can walk way out with a platform at the end and ladders to get in/out of water. We did not purchase the Mahogany Bay chairlift pass. It really is not a very long walk to the beach from the ship, plus it's shaded for much of it with pretty resort landscaping (see pics/map... https://www.mahoganybaycc.com/port-information.aspx) https://www.carnival.com/shore-excursions/mahogany-bay/extreme-zip-line--beach-adventure-518090