By Public Transportation
Buses do not actually go into Old San Juan, and one walk across the cobblestone roads will show you why. However, they do serve a major bus station on the harbor side of the area, near one of the major cruise ship piers. There is a free, often-running trolley that takes people around the forts, museums, and the center of Old San Juan. Trolley stops are marked with yellow banners that show an illustration of a trolley and a number that indicates which stop it is. You can get on or off at any designated stop, and the trolley driver will help you find your stop if you aren't sure. If you're feeling a bit warm in the midday heat, hop on a white trolley which is open on the sides to catch a nice breeze while speeding to your destination. On Saturday and Sunday afternoons, when cruise ships bring in hundreds of tourists, the trolley is often completely full and does not pick up new passengers. Try to go early.
By taxi
It is $20 to go within Old San Juan by taxi, and this option may be best if you are not practically dressed or not in comfortable walking shoes, or traveling as someone or with someone who cannot walk long distances easily.
By car
Old San Juan is primarily made up of one-way cobblestone roads, and parking is practically non-existent. If you must drive, try to drive and park along Calle de Norzagaray or any other streets on the northern side, and park there if you can. It is near the main attractions and the trolley system which can take you into Old San Juan and out back to your car when you are done.