Fun (expensive) shopping, and yummy French pastry and chocolate; however, stereo-typically unfriendly French locals. BTW, I've been to France on several occasions (including Paris multiple times) and the 'real' French are always friendly.
We did take the bus tour (from Philipsburg on the Dutch side around to St. Martin on the French side and back again). It was an odd combination of natural beauty, industry and poverty. The stop in the St. Martin capital of Marigot was the best as we did some 'local' shopping there. The downside of this port was that we visited on a Sunday and much of the island was closed for the day.
Once again, here we took the organized excursion in the form of a bus tour. It went all the way to the top and back down again. While this afforded amazing vistas and picture taking opportunities, it was longer than we liked (over 2.5 hours) for what it offered. Also, there was a stop at Myett/Cane Garden Bay that was clearly set up as an 'opportunity' for the cruise tourists to spend money in this isolated spot. I could have gone without that stop and been perfectly happy with the rest.
Actually, I don't think we went to 'Dominica'; however, our stop wasn't on the list of options. The port was Cayo Levantado in the Dominican Republic. We were offered only two options at this port: 1) Independently go to Cayo - essentially a private beach for cruise passengers or 2) Take an organized excursion in Samana. Apparently, it isn't safe to run around the DR unattended.
The island was an excellent choice and we fully enjoyed it. It was beautiful and included an open bar and BBQ. The shopping 'mall' on the way in was a bit heavy handed, but we still picked up a few fun items and truly enjoyed this port.
This would have been our least favorite port had it not been for the Atlantis portion. 100% touristy (felt a bit like Vegas); however, it was pretty and did provide some entertainment.
We also started with a bus tour here (really nice driver, but nothing of interest to see - the 'forts' were about it and they were nothing to write home about) and got off at Atlantis and made our own way back to the ship.
Caution here: If you depend on a taxi to get you back to the ship, your options are limited: 1) Wait for 15 people to gather and shove themselves into a taxi and then pay $4 per person or 2) Take it on your own for $15.