The Brecha de Rolando (2807m) is a breezy 40m-wide gap in the mountain wall along the French frontier, providing a dramatic route to/from the Cirque de Gavarnie. The direct approach from Pradera de Ordesa is absolutely not for the vertigo-prone, while the main alternative route, from the Refugio de Góriz, includes an exposed section (with a metal chain to hold on to) across a not-quite-vertical slope.
The Pradera-Brecha route (about 6½ hours one-way) involves climbing the Clavijas de Cotatuero, a set of 32 iron pegs hammered into the rock wall of the Circo de Cotatuero. The usual Góriz-Brecha route (about four hours) includes the Paso de los Sarrios, a 50m stretch across a vertiginous slope just south of the Brecha – there's a chain to hold, but if you prefer, you can circumvent the Paso by heading downhill before it then back uphill after it.
The French Refuge des Sarradets (often full) is about an hour's steep hike below the Brecha on the French side (crossing what's left of a glacier, frequently passable without any special equipment except poles).