Built to house a relic of the Buddha, Wat Mahathat is one of Bangkok's oldest shrines and the headquarters for Thailand's largest monastic order. It's also the Center for Vipassana Meditation, at the city's Buddhist University, which offers some programs in English.
Adjacent to it, between Maharaj Road and the river, is the city's biggest amulet market, where a fantastic array of religious amulets, charms, talismans, and traditional medicine is sold. Each amulet brings a specific kind of luck -- to get the girl, to pass your exams, to keep bugs out of your rice stock, or to ward off your mother-in-law -- so if you buy one, choose carefully. (The newer amulet market is part of Wat Ratchanada, off the intersection of Mahachai and Ratchadamnoen Klang roads, across from Wat Saket.)