I recommend just using the services at the hotel. The ones not capable of doing walks etc, can stay at the hotel and enjoy themselves while the ones who's a bit more adventuous can take a cab on your own to Old San Juan and walk it on your own, just up and down streets, shops etc.. and if you're going to do the fort, pay for just one. At the entrance, you can buy a pass for 2 fort and save a little bit on the second one, or just buy tickets for one. By the time you're done with the first one, from all the steps... most ppl won't bother with the second one, and takes time to walk the fort as well.. forgot the name now but we went to the bigger one. By the time you disembark, walk around in old san juan, rest a bit in the hotel, I doubt if you still have a lot of time to do anything else.
Not sure what time is your flight, but get to the airport early, give yourself AT LEAST 2 hours, and it is more than a zoo from what i experience. If you're on USAirways.. more power to you!! They're renovating and it's utter mess, total disaster.
Also, have you thought of walking old san juan on your embarkment? The cruise ships are just docked in old san juan, some within walking distance, some, not but that can save you some time to visit other places in san juan on your return. If you're docked on the Amercana's side, you can actually take a bus to town, only a quarter i think instead of spending 10 bucks on a cab. you get of the pier, cross the road and ask some one or look for bus stop. Buses should be heading right and quite frequent, we waited about 10 mins if that.
Alcohol at the san juan port is one of the cheapest.