Itinerary for North of Spain/South of France in 3 weeks

Instead of Carcasonne, consider lovely waterfront Collioure for your overnights. You could still stop on Carcassonne en route.

Central Spain will probably be very hot during your timeframe, but if you can tolerate it, I'd toss Toledo into the mix.

I enjoyed the Spanish Basque country a lot and think it's a great place to spend time in mid-summer. It will probably be much, much cooler than most of your other destinations. I liked Bilbao even better than San Sebastian, and Vitoria-Gasteiz is also nice. There are a bunch of attractive coastal towns/villages, starting with Hondarribia.

Unless you hit the running of the bulls, Pamplona is really not very special. I consider it totally skippable. I think you'll find guidebooks are very lukewarm about it.

On our last trip to Spain we entered in S/S and flew out of Barcelona.

Several comments.

Skip Carcassonne unless using it as s base. We spent 3 nights. The old walled city is overhyped kinda Disneyesque. We spent one very nice day Cycling along the Canal d Midi and another ona one day loop south through the cathar castles -2 in particular. Quite amazing.

We did Bordeaux as a day trip from a farm stay east of St Emilion. It was nice but ...

Instead of Pamplona consider LaGuardia - a delightful hilltop village that is a good base. We did a day trip to Pamplona which we enjoyed; and another day we rented bikes and did a section of the Camino trail including a winery stop.

And finally one place that was a real joy was Gerona ... a fast train ride NE of Barcelona. We stayed in Barcelona (4nights) and 3 in Gerona.

A plus one on Toledo - a more interesting place IMO than Segovia.

I mean all of the places you mention are nice - just seems a bit rushed - but however you slice it it will be great!

Overall I agree with Daniel on most of his comments.

Carcassonne is overhyped, the time spent including it could be better spent.

Unless you are big wine fans, the time spent going to Bordeaux could be spent on the beach in Biarritz with your toddler.

With Pamplona in July, be careful. The 6th is the official start of the San Fermines. LaGuardia is nice, but you might do better going from Segovia to Vitoria.

I would substitute Bilbao and Provence for Madrid+Segovia+Pamplona

Start in Barcelona (5)
Fly to Bilbao (2)
Bus to San Sebastián (3)
Bus to Biarritz (3)
Train to Bordeaux (2)
Train to Arles - Provence (4)
Train to Barcelona (1)

I agree with Daniel’s assessment of Carcassonne, except he neglected to mention the myriad souvenir shops, selling kitschy tourist trash. I felt lukewarm about Bordeaux, but nearby Arcachon is great. If you’re going to Biarritz, you should consider some exploration of the Basque villages in the vicinity. There is a little train, Petit Train de la Rhune,, that you can ride from Ascain to the top of La Rhune, where you can see the pottocks, a wild pony, native to the area.
We have never had a problem renting a car in one city and returning it to another, within the same country. Trouble arises if you want to drop off in a different country, in the form of huge fees.