I would take Rick's advice if I were planning this trip. If you have one night in Pamplona and one in Santiago de Compostela, that leaves two nights in between. Here are a half-dozen choices, cut 'n pasted from Rick's list, to choose from in that section:
▲ Burgos Sprawling but walkable city centered on its glorious Gothic cathedral, with loads of quirky touches.
▲▲ León Bustling city with grand Gothic cathedral (crammed with Spain’s best stained glass), fresco-slathered Romanesque chapel (in the San Isidro Museum), and lively tapas-bar scene.
▲ Astorga Pleasant town graced with Antoni Gaudí’s visit-worthy Bishop’s Palace.
Villafranca del Bierzo Sleepy pilgrim town perched on rugged hills at the edge of Galicia.
▲▲ O Cebreiro Quintessential Galician mountain village, with stone palloza hobbit houses, a pre-Romanesque church, and oodles of pilgrim ambience.
▲ Lugo Atmospheric Galician city just off the Camino, lassoed by stout, mossy walls.