Check out a Fodor's tour guide book for Puerto Rico from your local library.
They have a historical walking tour of Old San Juan that was perfect for our family. We just did it last week. It starts at the pier and takes you all around the waterfront through the gate and to El Morro, the one fort and then leads you to the other fort San Cristobal with famous plazas, churches, fountains and other historic sites along the way. The end of the walking tour is just a few blocks away from the pier so it is perfect for cruisers.
Entrance fee of $5 for each person 16 and over gets you into both forts and you can beat the crowds by taking your walking tour first thing in the morning. Overall the walk took about 4 hours with us taking our time with lots of pictures along the way. It was perfect for our family and we thoroughly enjoyed Old San Juan in the morning without the throngs of cruise passengers that were on the streets when we were heading back to the ship.
Best and cheapest way to get a taste of San Juan $10 bucks for our family of 4 and uncrowded streets ( at least for the first 2 hours) Can't beat that.