Few places in the Americas hold such a repository of colonial architecture as Antigua does. Some still serve their original purpose, whereas others have morphed into hotels, restaurants, shops, and homes. Still others stand magnificently in ruins. If ever there was a place you could overdose on history, Antigua is it. Avoid the temptation to rush from church to convent to monastery to palace and off to another church, all in one morning. (You'll see dazed tour groups doing exactly that.) Build in some down time and delight in the small things (ice cream, music, or shoe shines) that make this one of the hemisphere's special places.
Nearly everything of interest (sights, restaurants, hotels, and services) sits in a 10-by-10-block grid in the center of the city, and you could possibly narrow that down even further by focusing on the immediate orbit of the central park. Most of the sights lie east, south, and north of the park.