Parque Los Dinamos sits at the edge of the city, but it feels like another world. The Magdalena is Mexico City's only river that still runs free. No concrete, no channeling, just cold water cutting through oak, fir, and pine forest high in the mountains. You'll take a car from a central meeting point, then hike 10 km along the river with a seasoned naturalist who reads the forest — birds, plants, insects, geology — and adapts the pace to you. Transport and entry are included.