North Spain for 9 days with three teens (15,17,20) in July 2015. What are the must visit cities?

You might want to move this post to the main Spain page of the forum, instead of the reviews section. See http://www.ricksteves.com/europe/spain for a good start on Rick's must-see list, but note that this summary doesn't include everything that's actually in the guidebook, such as Bilbao. When you say north Spain, I think of just the north coast. If you also mean Madrid, Barcelona, and everything in between, then add those details on the next post.

Agree with Laura. Your post is very vague and hard to give much advice. Northern Spain is the Basque Region (such as Bilbao), Castile/Leon (Valladid), Rioja and maybe northwestern Spain (Galicia). I would not consider Barcelona (Catalan) or Madrid to be northern Spain. Where in Portugal? Lisbon and Porto are the most common destinations, but with teens you may want to throw in a beach destination along the Algarve (which is the south coast). Barcelona is very popular with young people and your kids might well really enjoy a few days there (it also has a beach). But Barcelona is at the very far northeastern tip of Spain.

Whether to rent a car or not depends on several things. If sticking to major cities and spending several days in each, a car wastes money sitting in parking lots (in themselves expensive). If flying into Spain and out of Portugal (to avoid backtracking), a car would have very steep foreign drop-off fees. But on the other hand, you might save money with 4-5 people in a car versus that many train tickets. But cars won't be very big so it might be too tight to fit 5 and luggage. Also, the Iberian peninsula is very large and distances can be long between major cities. Yet not all Spanish/Portuguese cities are well connected by train. A car might be useful on a limited basis if exploring a region. I rented a house once that had parking, then used a car to take daytrips in the region. These are general considerations, once you have a better itinerary, then you can figure out transportation.

Thank you both for your comments and/or suggestions. I will move my post and the vagueness of my question is due to my lack of information. The answers so far have helped me and I will reconstruct my post once I have a better idea of our trip. Thank you for taking the time to answer me!