Camino Norte, Camino Ingles and the main Camino Frances all pass through Northern Spain. Other routes also obviously converge on Santiago de Compostella. Camino Portuguese is supposedly very beautiful and relatively easy, but it is the only route I haven't walked.
90% of walkers head towards Santiago, but many people walk 'backwards' away from Santiago. The sanderos (paths) and albergues (hostals) are used by non-pilgrims also. It is popular with mountain bike groups.
IMO all of the routes have some very magical moments. Camino Norte follows the coast before cutting inland. It is breathtakingly beautiful much of the way. Camino Ingles is a short route - also stunningly beautiful. The main camino (Frances) can get busy during high season, so don't rely on the cheap/free pilgrims albergues.
To obtain your credential (pilgrim passport), simply ask at a church, or cathedral where to go. €1 gives you access to cheap/free accommodation. Collecting the stamps is fun also. There is a great sense of comradery amongst walkers. There is no 'hard sell' on the religious front. You meet people from all sorts of backgrounds - many people I met became good friends.
Sweet Meiga posts here. We met initially as 'pilgrims'. She may have more advice.
Leon in Northern Spain is my second favourite city in Spain. Worth a couple of days and an overnight stay.
For further info go here:
https://www.caminodesantiago.me/
As far as non-walking goes...
Buses are good value and very comfortable.
Blah blah car is extremely popular in Spain.
Trains are either very fast and modern, or very slow, but charming narrow gauge commuter trains.
Euskotren is worth checking for meandering through valleys and mountains in Pais Vasco. Hop on - hop off at interesting little towns nobody knows about. There is also a small train service which runs pretty much along the entire coast from Biarritz. Great for coastal views, or visiting places like Guernica. You could use this train to travel to Biarritz and catch a connection to Bayonne for other mainline services around France.
In France there is a main GR route to follow across the Pyrenees. If you go that way be sure to visit
Pau. France is around 40% more expensive for hotels than Spain.
I could go on forever. It is all very beautiful and varied.
Ask any specific questions. Don't pop by here often ATM, but I do check a couple of times a week.
Enjoy it - you are going to have a fabulous time