Bilbao to Cahors - what to see & where to stay

Not having been south of the Lot in SW France, we are travelling to Bilbao early May, for about a month, to explore northern Spain  and the bottom west corner of France. At the end of the period we will be staying with my sister near Cahors hence the odd route!

Suggestions for places/sites in northern Spain would be welcomed as would those in France on the route Lourdes, Carcassonne, Beziers to Cahors.

Thanks, Steve.