Portugal and Spain

I suggest that you get a couple of good guidebooks, including the ones that Rick Steves has. Then do a lot of reading on this forum plus the TripAdvisor Portugal and Spain forums plus the Travel Portugal Facebook page. Most of your questions have already been answered.

It's hard for us to suggest anything when we know nothing about who "we" is, what your interests are, do you plan to drive or take the train, what kind of budget you have, etc. etc.

EDIT, I just looked at your posting history, and see where you said that you will be spending nine days in Spain. Is this still your plan?

The best itinerary depends on you and what you want to see: Lisbon, Porto, Sevilla, Madrid, Granada, Cordoba, Barcelona,...
My opinion would be to limit yourself to 4 cities spending about 4 nights in each location you choose. 5 or more would be a little too rushed and you would be spending a lot of time traveling, packing, unpacking and changing lodging. My 2 cents opinion would be Lisbon, Sevilla, Granada and Barcelona.

There are limited options for crossing the Spanish/Portuguese border on the ground. There are buses from the Algarve (including Faro) to Seville, and there are trains from Porto up to Galicia, the area where Santiago de Compostela is located. For a trip visiting the classic destinations in those two countries (some combination of Lisbon, Porto, Barcelona, Madrid, Cordoba, Seville, Granada--which altogether is too much for 17 days), you're probably going to be stuck flying. Personally, I hate having to deal with airports in the middle of a trip.

Many people who start out planning to combine Spain and Portugal on a single trip decide not to do that when they realize how limited the ground-transportation options are.

Don't have much time to comment so just two quick thoughts on Spain:
Cordoba (the mosque, and for something different, the riding school demonstrations just down the street a ways)
Toledo (walk up to the parador before sunset and watch the sun go down and Toledo light up. Wow.)
OK: the pueblos blancos: Ronda
And Granada's Alcazar.
there. that does it.

We just returned from a 19 day trip to Portugal. Highlights were Sintra and the Douro Valley. We spent two nights in each place and could easily have stayed longer. A distant third was Porto. (Sorry, but Lisbon didn't make the cut!) In Spain, our favorites are Seville, Toledo and Salamanca. Some of the other cities like Barcelona and Granada had such crowds at the major sights, it took away from our enjoyment of the place. Final thought . . . we suggest you spend more time in Spain than in Portugal.

17 days isn't enough to see both countries.
My favorite part of Portugal is Porto and the Douro River Valley.
For Spain, if you leave from Barcelona, you need four days for that city. Don't miss Madrid, Toledo and Segovia in Castile. Seville is another jewel of Spain. You will have a tough time deciding what to cut with only 17 days.

We (husband 67 and me, 61) spent May 7-June 8 visiting 5 cities in each country last year. This was our first trip to Europe. I did a ton of research using RS books and this travel forum. It was a lot of work, but fun. By the time we left home I had our itinerary planned and our "must-see" tours/activities tickets booked/printed before we left (ie Alhambra, Mezquita, Porto Food tour).

I also booked/printed our travel tickets on buses and trains beforehand. The Flixbus from Lisbon to Lagos was extremely comfortable and the rest stop on the way was definitely 5 stars compared to the highway rest stops we've encountered here in the US!
Having these transportation arrangements made in advance saved us time/mental energy and allowed us to enjoy our trip even more without having to try to figure it out on the fly or in the hotel. The buses and trains left us envious for such systems here!
Have a great trip!