The main hike from the Centro de Visitantes is Cerro Bahía Blanca (two hours round-trip). There are great views from the trail and the peak and you don't need a guide to get there. There are also Cuevas con Pinturas Repustres, or cave paintings, but you can only see them via a guided tour with an agency out of Sierra de la Ventana.
The park’s highlight, the five-hour (round-trip) guided hike to 1150m Cerro de la Ventana, which leads to a window-shaped rock formation near its peak, is accessed via the westernmost entrance. The climb offers dramatic views of surrounding hills and the distant pampas. Register with rangers before 9am at the trailhead, and take plenty of water and sun protection. That hike costs an additional AR$140 per person.
Less-demanding destinations accessible from the Cerro de la Ventana trailhead include Piletones (2½ hours round-trip) and Garganta Olvidada (one hour roundtrip). To visit the gorge and waterfall pool at Garganta del Diablo (AR$ 200 per person, six hours round-trip), you need a guide, arranged by rangers at 9am at the trailhead, or you can book with a tour company out of Sierra de la Ventana.