You can walk Spain's favourite trail, the 10km Ruta del Cares, in either direction, but head from north (Poncebos) to south (Caín) to save its finest stretches for last. The beginning involves a steady climb in the gorge's wide, mostly bare, early stages. From the top end of Poncebos, follow the 'Ruta del Cares' sign pointing uphill 700m along the road. After 3km, you'll reach some abandoned houses. A little further and you're over the highest point of the walk.
As you approach the regional boundary with Castilla y León, the gorge becomes narrower and its walls thick with vegetation, creating greater contrast with the alpine heights above. The southernmost stages of the walk are possibly the prettiest, and as you descend nearer the valley floor, you pass through a series of low, wet tunnels to emerge at the southern end of the gorge in Caín.
If you're feeling fit, it's quite possible to walk the whole 10km and return in one (somewhat tiring) day's outing; it takes six to seven hours plus stops. There's no drinkable water along the route, so bring your own.