We knew that the four ports of call for our January 2012 cruise through the Panama Canal would be hot and humid, and we were not interested in paying for ship excursions, particularly the "adventure" ones. Our goal was just to walk around and try to get a feel for each port.
You can easily walk from the dock to the port town in Aruba, but there isn't much to see there beyond the usual souvenir shops. However, Cartagena (pronounced cart-a-hay-nya) has a brand new catamaran service from the end of the dock to the "old town," with its narrow streets and Bouliva Plaza with its free gold museum. The trip costs $5 per person round-trip and takes about 15 minutes each way. An excellent independent excursion.
Fuerte Amador is a great disappointment. If you don't want to take a cab to Panama City (and note that museums usually close on Mondays), you can take the tender to the dock and walk a little. The docking point is at the end of a 5 km sea wall constructed at least a hundred years ago to protect the Pacific entrance to the Canal. There isn't much of anything to see there except a few run-down shops and restaurants.
Puntarenas, Costa Rica, has a mile or so of shops and stands, the people are extremely friendly, and there are actually some bargains to be had. (Note the beautiful rosewood bowls and boxes!) The dock is perhaps a hundred yards long, and the stroll along the main street is interesting.
If you are used to finding internet cafes to check email at reasonable prices, be advised that ones with computers to rent are getting hard to find. We did find ones in Aruba and Puntarenas, but generally you'll have to bring your own and use a cafe or other place with "hot spots" that allow you to connect to the internet.