Ushuaia is Argentina's southernmost town, and has a fascinating landscape of windswept barren lands, beautiful blue glaciers and national parks. The town offers fine views of the Beagle Channel (so named after the boat in which Charles Darwin sailed through it in 1832) and the snow-covered Cerro Martial towards which the streets steeply climb.
Some hotels will let you use their facilities (gym, spa, pool, beach, etc.) for a reduced day pass. This can be great for layovers!