Spain-Basque Region to Portugal

1. No way to know for sure. You can look at historical data, and that will help, but you can always get "lucky" and be hit with an abnormal heat-wave (or cold period). Especially recently, it seems "unusually hot" is becoming less unusual across many parts of Europe in the summer.

2. Check rome2rio, but I am not aware of any good (direct) train options to cross the border in that corner of Iberia. Then again I haven't looked carefully for a long time, there might be. If you can find two towns with car rental agencies, which are near enough the border (on opposite sides) then you might find a short bus could connect the two. I did that once at the other end of your trip - dropping one rental car on the Portugal side, crossing into Spain (by a combination of foot, small ferry and bus) then picked up a second car in Spain. Was a bit of a hassle and time consuming, but saved that big rental car drop-off fee.

3. Sorry can't remember towns we drove through - much of the landscape is spectacular though (reminded me of the coast of the US pacific Northwest). That's a corner of Iberia I long to return to.

How old are the boys, how much time are you allowing to get from San Sebastián to Porto, and which route do you intend on taking? Not far from San Sebastián is Santillana Del Mar. it’s very small and touristy, but is like walking back in time. If the boys like beaches, then there are plenty of those along the northern coast. Cathedral beach, west of Ribadeo is great if you can get there at low tide. We also liked the coast at Muxia. Larger cities worth visiting include Leon, Zamora, and Santiago de Compostela. If the boys like castles, there are nice ones in Ponferrada, Medina Del Campo, and Puebla de Sanabria, but I’m sure there are more. Covadonga in the Picos de Europa is nice and the nearby town of Cangas de Onis is nice to walk around. Those are some places in that large area that might be of interest.

Yes I would get AC in Biarritz in July.
You could pick the car in San Sebastian drop it in Vigo, then bus to Porto airport (many bus per day) then take a rental car for Portugal.

Depending on your boys ages, there are many nice hikes in Picos de Europa.
Take the cable car in Fuente De, then hike to the refugio.
Cares del gorge is another nice one but not with young kids. Covadonga is worth a stop if you like nature. We stayed a few days in Llanes and spent a few days at the beach and a few days hiking in Picos de Europa.

Hope this help!

Thanks for all the recommendations. I don’t have a route chosen yet, and we don’t have to be in Porto in any particular date as we have a month to explore.

I live in Bilbao, fairly close to Biarritz, and nobody that I know has A/C at home, we practically never need it, it´d be an eccentric extra expense. Temperatures are quite mild in summer, in the 80´s maximum, rarely over, and you´ll most likely need a sweater at night. Sea temperature around 70 Farenheit. Moreover, this is a rainy area, so bring raincoats just in case. Wondering why Santander (it´s Cantabria, not Basque Country)? Not much to see there, there are a couple of beaches, but again, beach weather is not always a real possibility.

Santander has a magnificent beach (or beaches?). It's not a bad city at all, but it suffered a catastrophic fire in the 1940s and thus has little old architecture to enjoy. From that perspective it is one of the least interesting Spanish cities I've stayed in--though Vigo is certainly duller--and I've spent 4-1/2 months in Spain in the last few years. Unless you have a special reason for staying in Santander, I would head elsewhere. Refer to jaimeelsabio's post for some ideas.

There is bus and rail service across the border between Vigo and northern Portugal, but check the schedules carefully; you could end up with a long layover at a transfer point--probably the aforementioned unexciting city of Vigo.

Agreed...Vigo has nothing to offer and it´s all up and down all the time, it´s built on a hill. And Santander is just nice because of the beach and the bay, but the city is not interesting (I like the food market, though).

I was in northern Spain a few times over the last 10 summers, including in Santander, San Sebastian, Vigo, etc. My advice is to get a room with air conditioning if you'll be there in June, July or August.