To add to Gettingwarmer's list we have been to San Juan and just looked back on the photos and that looks Ok to walk around with a wheelchair.
Dominica - Roseau town is a bit run down and I imagine would be difficult with the state of the kerbs and pavements. It is a nice island if you can get out of town, there is a good botanical gardens not far out of town that overlooks the port that is good if you can get a taxi to take you up there.
St Maarten - it is about 15-20 mins walk into town. There is also a boat that runs from the ship to town but not sure how practical that would be.
Willemstad Curacao - As Gettingwarmer says this would be a good one to just walk around the port, easy walk into the town with pretty coloured buildings.
Ochos Rios - echo what Gettingwarmer says, even if you can dock it is not a very nice place to walk around.
If you can use taxis - In most of the Caribbean ports there will be taxi drivers who will take you around for a tour, they usually tout for business as soon as you are off the ship.