I visited this WHS in May 2014. I spent 3 days hiking in the Ordesa Valley and I saw Mount Perdu both from strenuous uphill trails like Faja de Pelay at high altitude (not suitable for those who don't have a head for heights) as well as from the easy trail in the valley with lovely cascades, streams and waterfalls. I must say that the most picturesque hike is indeed the easy hike by the Rio Arazas which takes about 7-9 hours if you stop for lunch and to take lots of photos. If you start early at around 7am you will beat the crowds and it really is a very enjoyable trek with rainbows forming early in the morning over the many cascades and waterfalls. The UNESCO plaque is immediately visible after a few minutes of hiking. The Cola de Caballo waterfall is beautiful but you should arrive early there as it's also the place where everyone stops to have lunch. I was surprised that I didn't have to pay an entrance ticket of any sort. All you need there are good hiking shoes and an empty bottle to fill with pure water from the gushing streams. During 3 days of hiking I spotted the rare Lammergeier (Bearded Vulture) amongst different birds of prey and scavengers as well as smaller birds. I was lucky to spot a couple of marmots and 2 big castors near Cola de Caballo and I also enjoyed the varied mountain flora with different kinds of butterflies flying freely around. This is truly a magnificent WHS and one of my favourite transnational mixed WHS in Europe.