If you want to 'tick off' as many countries as possible, I'd concentrate on the smaller ones (Belgium, Luxembourg, Netherlands, Denmark, which will necessitate driving through France and Germany, once you're in Denmark, Sweden is very close).
However, I don't think you should try and go as far as Spain as well, won't you have a nicer holiday with less driving and more relaxation and experience of local attractions? So either France and northern Spain, or northern European countries, not both.
We drove to northern Spain about 30 years ago and I'd love to do it again, but I could fill a month or more to just drive down through France, stopping at a few places on the way, south of France, through the Pyrenees, along the Costa Brava, Girona, Barcelona and back up to northern Spain stopping at a few places along the way, Basque coast, San Sebastian, Bilbao, Guernica and back over the Bay of Biscay from Santander.