You can very easily explore Old San Juan on your own, and a guidebook will offer much of the information you need. Alternatively, if you're really interested in an in depth visit, you can book a three hour tour for $45 a person. It's quite expensive, but it is well done and informative. You'll get a guide that is usually very knowledgeable and entertaining. You begin the tour on La Princesa Boulevard which you reach by bus. You walk through the old town passing fountains and the old city wall. You'll see the Rogativa Monument and a variety of old churches and historic buildings. It can be brutal to walk for three hours in the tropical heat, but it's a great option if you really want to understand more about the city.