On the slopes of La Peña Montañesa, San Victorián monastery has claims to be the oldest monastery in Spain. Dating from the 11th century (though much modified between 1593 and 1613), it was deconsecrated in 1953 after suffering severe damage in the civil war. Recently restored, it's open for guided tours during which you can see the main buildings, the cemetery and bits of earlier churches uncovered by archaeologists, one of which dates from the 6th century.
The monastery is a 19km drive northeast from Aínsa, either via Arro on the N260 or via the quaint old stone-built hamlet of El Pueyo de Araguás.