Well, one of these days I'm actually going to write our trip report from the 2019 WBPC.
That said, and these are what we did, not necessarily what would be the most popular, just how our ports were and what was good/not great.
Grand Cayman-- we didn't make decisions very early what to do and at the last minute were going to go snorkeling, booked through
DCL on board. However, when we got off the ship, the guide said there was, quite a bit of jellyfish that day (which is not necessarily the case every day), so we had the option to cancel. We weren't interested in a stingray excursion (I've done that already, and find it totally cool, but DH is not a fan). So we ended up on a small tour bus that we hired at the port area, hitting a few highlights: Hell, a rum cake store, a quick look at the dolphin experience area, and a beach (that we could have stayed at and hired a taxi back, but we just decided to get back to the ship). It was so-so, and I wouldn't specifically recommend it, but we got to see a bit of Grand Cayman with the benefit of air conditioning.
If we ever go back there, I might be interested in the turtle center.
Also, I did enjoy snorkeling with stingrays on a trip years ago-- my brother had booked that independently, and we were out in the water (about 10-15 feet deep) snorkeling around them, not in the shallows where I think a lot of the stingray experiences take place.
Cartagena-- we did a tour via Dora of Cartagena, booked independently. Had a great time seeing city highlights. Yes, they give you a little shopping time, and the street vendors are plenty, but we enjoyed our tour guide and were happy to see around this city, which has some pretty and charming areas and lovely architecture.
Panama Canal was, of course, the highlight of the trip. We were up on deck almost all day, except for a nap mid afternoon (when we were not in the locks).
Puerto Vallarta-- We booked the Las Caletas excursion through Disney and really, really enjoyed it. Food and beverages provided, went snorkeling, took a cooking class, and relaxed in a waterfront hammock.
Cabo-- whales were there this time of year, so we went on a whale watching boat (booked independently of DCL). We only saw a couple of whales, but the boat ride in and of itself was quite relaxing, with food and beverages provided. I wasn't in charge of booking it, so I forget what company it was, but they were very professional and it was a good excursion.