The historic heart of Manila, Intramuros (literally meaning "within the walls") is the oldest district in the capital city. The three-mile-long stone wall completely surrounds the district (with the exception of a small stretch near the River Pasig) and despite the fact that the district was nearly entirely obliterated by US bombers, it remains for visitors a rich cultural experience (with the help of ongoing restoration efforts).
Historical attractions within Intramuros include Fort Santiago, Postigo del Palacio, Baluarte de San Diego, Puerta de Isabel II, Plaza de Roma, San Agustin Church, and Ayuntamiento. The church, which was the sole structure left unmarred by the bombers, has been declared an UNESCO World Heritage Site There are also multiple museums and other points of cultural interest, many ruins, and a few tasty eateries. Intramuros is also a great locus for buying souvenirs and local wares. There are also a few choices for overnight stays, and somewhat surprisingly, a golf course.
The easiest way to access Intramuros is by taxi. The city's cabs are fairly affordable and easy to flag down, and they can take you directly to the entrance. You can also take a train to the Central Terminal, though it is still a fairly long walk from the western end, so you'll want to wear appropriate footwear. There are numerous walking tours available, all of which can help you learn more about this historic area. Access to the district itself is free and open to all, but some of the aforementioned attractions (Fort Santiago and San Agustin in particular) charge a nominal admission fee.