In the Pyrenees, the GR10 is a 500 mile route along the northern slope from the Mediterranean Sea to the Atlantic Ocean. It is a well-marked route with scenic villages and passes through alpine scenery. There are numerous stream crossings and lots of climbing, so you may want to consider taking hiking trekking poles.
Along the way, there are numerous huts for lodging, but camping is also permitted. If you do want to make use of the lodging, it is recommend to make reservations about three months in advance.
If you are looking to sample a section of the trail because you do not have the time for 500 miles, try the 82 miles section from Ste-Engrace in the west to Cautrets in the east, which crosses two Basque provinces. Make sure to stop in the town of Lescun and also in Gabas (stay at Refuge de Gabas).