Burgos, Leon and Santiago itself. Burgos cathedral is amazing (and see the museum) and it is a great city with fabulous restaurants in all the little streets in the centre. To my mind Leon Cathedral, with stunning stain glass windows (the "temple of light") is unearthly In its beauty. Unmissable in Leon is also the Basilica of San Isadoro with its extraordinary Royal Pantheon and C12th murals. You could spend several days in each and still find more to see.
Pamplona is a good rest stop but most people find it too soon. Useful for posting the stuff you don't need, buying stuff you do need, recovering from injuries because you started too fast.... Logrono if you need a rest and want the best churros con chocolate outside Barcelona (and a great little street of tapas bars).
And Santiago? Ahh. Words cannot describe. But you will definitely want a few days there to catch up with all those you have met on the road, and to celebrate.
