You're better off getting information on the Sacred Valley before leaving Cusco, either at the main Tourist Information Office (tel. 084/263-176), Av. El Sol 103, or at Cusco's branch of the South American Explorers Club (tel. 084/245-484). Your best bet for exchanging cash in Ollantaytambo is in small shops. If you need medical assistance, go to Centro de Salud, Calle Principal (tel. 084/204-090). The post office is on the Plaza de Armas.
When Not to Go
The Inca ruins at Ollantaytambo are often overrun with tourists doing the bus tour of the Sacred Valley on Sunday afternoons, typical market day. The ruins are so special and so enjoyably serene on other days that if you can't get there early in the morning on Sunday, it's worthwhile visiting another day, if you can.