Old San Juan is a beautiful place to visit. Just make sure you have a few hours to spare in order to absorb the culture, architecture and atmosphere. The city itself is a little hilly, so I would recommend starting out close to the ‘Paseo de la Princesa’. Here you will find a car park and roadside carts serving local ices (piraguas) and food. From here you can either jump on one of the local trolley buses (they are free) and take a ride through the streets up to El Morro (the fort) or if you are feeling energetic take a walk through the streets and head up to El Morro. If you do decide on the trolley bus, you can jump off at any time.
Old San Juan is full of narrow streets, so it might be wise to get yourself a map or at least download one from google maps or mapquest.
El Morro itself is definitely worth a visit, not just for the history aspects but for the views that you will see. If you go on a weekend you will most likely see locals flying kites in the grounds in front of the fort.
From El Morro head back into town and check out the Old Cathedral on Calle Del Santa Cristo (calle means street). Close to the Cathedral is the hotel El Convento which is a restored convent that is a good place to hang for drinks or even tapas. Another good place for breakfast or brunch is La Bombonera on Calle San Francisco. This is one of the oldest café/restaurants, its pretty basic but well worth a visit.
While wandering around the streets, check out; Plazuela de la Rogativa, Plaza de Armas and Plaza de San José where you will be able to grab a drink or a snack at one of the local café’s.
If you still have energy, then head to the Old San Juan Gate. The gate is close to La Fortaleza, the current official residence of the Governor of Puerto Rico. Head through the gate to see views of the bay and then take a stroll (left) along the Paseo de la Princesa. The promenade skirts the bay underneath the old stone walls of the city. This is an exteremely nice walk on a sunny afternoon or early evening. The promenade will take you back to your starting point, passing statues and fountains. If you are lucky you will also see groups playing dominoes or hear local musicians playing.
There are many café’s and bars throughout Old San Juan, so you will not be stuck for a place to eat or drink. Restaurants that have good reviews (from what I have read) are:
The Parrot Club on Calle Fortaleza 363
Dragonfly Calle Fortaleza 364
Amadeus Calle San Sebastián 106
Tantra on Calle Fortaleza, if you like Indian Cuisine.
There is way more to do in Old San Juan than what I have covered here, so head there and have fun, just remember to wear comfortable shoes.