Cueva del Milodón is the easiest great excursion from Puerto Natales. The main cave runs about 200 metres into the rock, big enough to feel like a natural hall, and the surrounding loop trails pass two smaller caves and open Patagonian scrub.
The cave is famous because skin, bones, and dung of a mylodon were found here in the 1890s. This extinct ground sloth stood several metres tall and lived in the region near the end of the Ice Age. The life-size model at the entrance is a little theatrical, but the place itself is genuinely atmospheric.