If you are in reasonably good shape, you can walk to El Morro as well. If your port call is a full-day one, I'd recommend including that on your walking "itinerary" if you choose to just walk around. The Old San Juan area is fun to see, lots of shops & little restaurants, historic buildings, cobblestone streets - lots of charm in among the "touristy" stuff. El Morro is a National Park Service historic site, and it's neat to see such an old building (parts date back to the 1500s) with NPS rangers in it!