We usually do it in the opposite direction, heading South.
Some of our favourite stops are:
Braganca. V Good free area (aire) [N 41.80390, W 6.74595]
Last year stopped at Chaves for a change. Parking only down by the river, but lovely town.[N41.7371, W7.469]
Valladolid (Spain) Another good area in a large carpark (was free for MH). V interesting town. [N 41.65606, W 4.73751]
Last year tried Leon instead. Similar. Less dedicated spaces. [N 42.60447, W 5.58423]
Vitoria-Gasteiz Another great town. Trams. Free area is again a reserved part of a large car park.[N 42.86595, W 2.68430]
San Sebastian. Paid for area. Busy. Easy busses into old town. Worth it. [N 43.30769, W 2.01417]
Next stop Bayonne.