For your visit one day should be sufficient, unless you are planning to hike the trails of Madriu-Perafita-Claror valley, which can be a multi-day adventure.
Worthy destinations: Andorra La Vella; Madriu-Perafita-Claror valley.
Last visit: August 2016.
Andorra is a little country wedged between Spain and France in the Pyrenees, that is unlikely to prominently feature on anyone’s must-visit list. There are a few skiing resorts of limited renown but very little in terms of attractions.
Andorra La Vella
The capital of the country is an agreeable enough town, with sufficiently picturesque historic Old City ♥ and some nice architecture in more modern areas. A few hours of exploration seems possible. Plaça de Rotonda is the focal point of the more lively part of the center. None of the churches or museums are of world quality; we only stepped into St Esteve Church.
The mountain landscape is what I came to Andorra for, since it is recognized as a World Heritage site by UNESCO. For those inclined to hiking, there is a wide field of exploration. Not suitable for people in indifferent physical shape, though. In my case of just a couple of hours of exploring, getting to a mountain stream was a highlight, but there are historic settlements and beautiful landscapes along miles and miles of challenging trails.