We are just off the Fantasy and did this excursion. I wasn't able to find out much ahead of time, but I wanted to share our experience simce there is so little info out there.
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DCL Port Adventure was great for our boys (9&6). My 9yo thought is would be good up to age 10, but older kids probably wouldn't get as much out of it.
We got a driving tour (15-20 min) on a motor coach, a guided tour of the fort (about an hour), and the kids program (50 min). The fort tour included locals portraying period characters, including Ponce de Leon, a General, a Soldier, and we were led by Spanish "soldiers" and a drummer carrying a flag. People were stopping to video and photograph our guided tour.
The kids program is in an air conditioned building next to the fort on Calle Norzagaray. It is led by a NPS ranger (we had 2) and several DCL counselors who come off the ship.
I talked to the ranger staff and learned that this is definitely NOT something you can do on your own, despite what the NPS website suggests. Disney basically rents it out for the day and runs the program jointly with NPS. The rangers generally do not lead programs here on other days and they do not accept drop ins on days the Fantasy is in port. While you could drop in other days if you were visiting PR or lived there, there is no organized program for drop ins. Parents must guide their kids and it is not set up like a kids museum.
All in all it was a great time. They provided us water and we could explore the fort or OSJ while the kids were in the program (we chose the fort). I would recommend it. All the kids seemed to have fun and everyone took home several certificates and a craft they made.