Check out Cabalgatas (tel. 01/9837-5813, www.cabalgatas.com.pe [1], US$45–65), an option for riding Peruvian paso horses near Mamacona, the town where the paso horse competitions are held each year. They lead interesting excursions around the ceremonial center of Pachacámac [2].
Links:
[1] http://www.cabalgatas.com.pe
[2] http://www.moon.com/destinations/peru/lima/sights/outside-lima/pachacamac