The tourist centre of Guam is just up the beach from Hagatņa. Called Tumon Bay, it's essentially a one-road-deep resort strip fuelled by hotels, clubs and restaurants. It's also geared towards Japanese package tourists, which translates into high prices.
Tumon Bay itself is quite shallow and at low tide it's possible to wade right out to the reef for a look around. Y'pao Beach Park is on the bay's southwest side and was once home to an ancient Chamorro village, as well as a leper and penal colony. These days it's a popular fiesta site.
Hagatņa (formerly Agaņa) has been the centre of Guam ever since the Spanish first set about remodelling the island on behalf of God. The town is small enough to explore on foot in an afternoon and there are plenty of parks and historic buildings, among them is Casa Gobierno, the Governor's Palace.
Don't miss the revolving statue of Pope John Paul II on the site where the man himself held mass in 1981; it puts the miniature Statue of Liberty to shame. In the southeast of the park, a statue of Chief Quipuha is condemned to survey the congested traffic of the main thoroughfare, Marine Drive.