Most people come for the water. The Valle del Mezquital is dry Otomí country, but it sits on hot springs, and the Grutas de Tolantongo — a warm turquoise river running out of a cave at the bottom of a canyon — is one of the great day trips from Mexico City. The park is run by the local ejido and you simply pay at the gate; the official site is grutastolantongo.com.mx, and it warns that other sites using the name sell fake all-inclusive packages.
Go on a weekday if at all possible. At weekends and holidays Tolantongo fills to bursting. Microbuses run from Ixmiquilpan, about fifty minutes away.
EcoAlberto, run by the Hñähñu community of El Alberto, has thermal pools, kayaking and rappel in the canyon of the Río Tula.