Chapada dos Veadeiros (about 220 km north) is one of Brazil's most beautiful national parks — a high plateau of waterfalls, canyons, and clear pools where you can swim — but it is more of a long weekend than a true day trip. Pirenópolis (150 km west) is a colonial gold-rush town with cobbled streets and a fine cathedral, three hours by car.
Closer to hand, the Águas Emendadas Ecological Station an hour northeast is one of the few places on Earth where two major watersheds (Amazon and Plata) meet from a single spring. The Catedral Rainha da Paz in Sobradinho, with a 32-metre fibreglass dome, is another half-day's outing.