El Chaltén calls itself the national trekking capital, and most of what people do here needs no operator at all: the trails to Laguna de los Tres under Fitz Roy and to Laguna Torre start from the edge of the village. What you book is the rest — boats on Lago del Desierto, kayaking, glacier walks and transfers up the Río Eléctrico valley.
For anything harder than a day hike — the Huemul circuit, the ice cap, a summit — you want a guide licensed by the national park, and El Chaltén has several companies of them, some going back to the 1990s.