Walk along a paved sidewalk with La Muralla (city wall) on one side and the blue Caribbean on the other. The walkway is lined with huge, old trees, street lamps, and benches. We started at the magnificent fountain/bronze statue near the docks where the cruise ships come in. The tourist office in the building across the street from the statue will give you a map of Old San Juan.
The view is absolutely beautiful. You can continue all the way to El Morro (and perhaps beyond), but you cannot get into El Morro from this walkway. You must enter the old city through the Puerta de San Juan. They were doing some much-needed maintenance at the old gate when we walked through.
This would be a very romantic walk at night. Though it is lighted, I don't know how safe it would be to walk after dark. Check with the locals.