21 days +- in Spain

If you were going during the summer, I’d recommend northern ES but since you’ll be there during the spring southern ES has the top sights. Fly in and out of Barcelona and Madrid known as a multi-city and visit Barcelona that can include day trips, Granada, Sevilla with a stop in Cordoba on the way, Madrid that also includes a day trip to Toledo. Instead of renting a car take the train: https://www.renfe.com/es/en. Here’s an idea…
Fly into Barcelona and spend arrival day adjusting to a 9h time difference and the next day hit the ground running. You need a minimum of:
Barcelona – three nights and if taking day trips add a night per trip.
Granada – two nights.
Cordoba on your way from Granada to Sevilla (store your luggage at the bus terminal behind the train station). After touring the Alhambra take a direct train to Sevilla for three nights.
Madrid – three nights but add a night per day trip i.e., Toledo.
The first thing you need to do is buy Rick Steves Best of Spain guidebook 3rd edition to decide where you want to go and what you want to see. You can download an electronic version from his app.

Hi, that sounds like a wonderful trip! You'll find the driving in rural Spain is pretty similar to driving in rural western US. If you are renting a car you are right to focus on smaller towns, where your own set of wheels is invaluable. Between Barcelona and Oviedo, I can recommend these points of interest (east to west):

Vielha - the capital of it's own autonomous community called Aran, which is tucked away in the corner of Catalonia. From there it's a short drive to the Vall de Boí, which is a mountain valley with many old 1000+ year Romanesque Churches, Vall de Boí is designated a UNESCO World Heritage Site.

Jaca - which is the old medieval capital of the Kingdom of Aragon and is dominated by a huge 16th-century star fortress. From Jaca, you can do a day trip to the Castillo de Loarre (about 1 hr), which is one of the oldest castles in Spain or explore the southern foothills of the Pyrenees.

Royal Palace of Olite - For me, one of the most impressive medieval castles in Europe. Set inside the walled medieval town of Olite, it was the old seat of the Kings of Navarre. There is a also pretty good Parador hotel next door.

Santillana del Mar - Well preserved medieval town in the Cantabrian region. Known for its medieval towers, Renaissance villas and the Romanesque Santa Juliana Collegiate Church.

Picos de Europa National Park - Mountain range between Cantabria and Asturias offers some of Europe's most spectacular mountain scenery. Make sure to stop at Potes, an atmospheric medieval mountain town.

Fuente Dé Parador - a historic hotel that sits below a string imposing vertical mountains of the Picos de Europa. From there you can take numerous nature hikes and even a cable car that takes you all the way to the top of the mountains.

Covadonga - known as the "cradle of Spain", a pilgrimage site dedicated to the Battle of Covadonga of 718 AD which set off the Reconquista. There is also a Holy Cave, where lies Kings Pelagius and Alfonso I, the first kings of Asturias.

Oviedo - The capital of Asturias. Known for its Gothic medieval old town, apparently Woody Allen's favorite city in Spain.

PS: I should warn you that April is the height of the rainy season in Northern Spain, so prepare to be flexible with your plans, you'll find some of the high mountains with snow still.

MaryPat makes a good point about the seasonal aspect, but as I get older myself, something I take into consideration now is doing things sooner that will be harder when I'm older and saving "easier" things for later when I'm older. Hence I'm trying to pack in the driving trips while I can! I am sure there are hundreds if not more small towns that would make a good route, but the first advice still holds--get some guide books and start filling in the blanks.
I think you'll find at that time if year you can wing it somewhat (with a skeleton plan and the Booking.com app).

PPS: Speaking of guide books, you'll find this one pretty helpful, it's so good I have a copy of it myself (even though I'm from Spain lol): https://www.dk.com/uk/book/9780241461532-dk-eyewitness-road-trips-spain/

I've never driven in Spain, so I don't have a lot to offer in terms of really small places. I liked Besalu and Ripoll in Catalunya.

Up toward the Pyrenees I enjoyed Puigcerda, which has some very attractive architecture, and took buses along the highway heading southwest, stopping off at a few old, small towns. There didn't seem to be other tourists about. All those places were very hot in mid-summer; I imagine they'd be nice in April and May. The town of La Seu d'Urgell has a pretty historic center.

San Sebastian is a bit larger than Santander. The historic area of San Sebastian is massively touristy; it seemed to me that virtually every building was a restaurant or pintxo bar. It's a pretty city, but...

Bilbao is almost twice the size of San Sebastian, but it is far less touristy. The historic center is populated mainly by locals, not tourists. Still, it's probably not what you're looking for.

There's some quite nice scenery in the Basque Country. If you zoom way in on the maps at ViaMichelin.com, the most scenic roads will be outlined in green.

There have been a number of posts about scenic coastal towns in the Basque Country by knowledgeable folks. You may find them by searching this forum for references to Lekeitio, Zumaia, Getaria and Zarautz.

Santander is quite a large city (over 170,000 people), and it suffered a massive fire in 1941. As a result, there's not much old architecture left. The beach is pretty; otherwise, I think you can do a lot better.

Santillana del Mar is cute but massively touristy. I'm not certain, but I think Spain's best cave art is near Santillana del Mar.

The Picos de Europa area is very scenic. My only option for seeing it was a once-a-week bus tour, so my exposure to the area was limited. I found the old stone town of Potes extremely picturesque, though not undiscovered. The area has some active-tourism options that attract visitors.

Thanks for the quick responses. Very helpful.

I do have four guide books for Spain including Rick Steves, DK Eyewitness Spain, DK Backroads Spain, and Lonely Planet Spain.
Sounds like maybe I should chek out DK "Road trips Spain"

Guide books are great but it is extra nice to get input from "real people".

Thanks again Mary Pat, Carlos valadelphia and acraven!

Your big problem will be narrowing down all the good choices--Carlos indeed makes everything sounds good. The part I have experience with is what acraven mentioned, but we did it primarily by bus: San Sebastian to the west. I remember the area from Lekeitio to Elanxtobe being especially scenic, even with the invasive Eucalyptus trees.

I've skimmed through a couple of the Backroads books, though much of their content isn't useful without a car. I think you may find that book very helpful.

A couple easy walks and small towns.
Alarcón - Sendero Hoz de Alarcón. A walk that goes around the city.
Teruel - Camino Natural del Rio Guadalaviar
Albarracín - Paseo Fluvial. Río Guadalaviar. Albarracín. Very small town.
Near SOS Del Rey Católico - Foz de Lumbier. SOS is very small.

We took a train from Barcelona to Pamplona picked up a car there and took 12 days driving back. We didn't feel that Pamplona was particularly big, very pleasant to walk around, two nites was enough. We especially enjoyed driving into the Pyrenees, staying in little mountain towns. Aragon, Navarre and Catalonia have more than enough charming little pueblos to keep you occupied for weeks. The driving was easy on good, lightly traveled roads. Ramble On!

Brad

I'm not much of a Pamplona fan. It gets quite a lot of visitors--I guess because of Hemingway--but it doesn't have very much historic architecture or all that much to see except during the actual running of the bulls. It's not a bad place (and the life-size running-of-the-bulls statue is cool), but I consider it (along with Santander and Vigo) one of the least interesting Spanish cities I've been to. I wouldn't choose it as a multi-night base unless geography (and I guess not having a car) dictated that decision. I much preferred Vitoria-Gasteiz.

Continued thanks for the additional input.

Richard