Stop overnight from Mont Saint Michel to Pamplona

La Rochelle is an attractive seaside town on France's Atlantic coast, largely due to its preserved old town and harbour fortress towers.

You may already know the Via Michelin site which helps you plan road trips. Generally, though, it underestimates actual driving times somewhat, and of course it doesn't take account of stops for gas, food, overnight, etc. La Rochelle would be a good choice, scenic and historic, and roughly halfway there. Bordeaux would be another option, but you may not want to navigate a big city on this long-distance trip.