Street art in Madrid. Take a walk around the older parts of the city when everything's closed (which is pretty often), and check out all the graffiti that people put on the closed shutters of shops. There's all kinds of interesting stuff, and the quieter, twistier alleys have the best of it.

don't tell the basques you're going to the basque "region"... if you don't like the word "nation" just skip any territorial references when speaking with [most, not all] of them, or just call it "Euskadi"... specially if you go to an 'herriko taberna' (basque bars)

not to alarm you but ETA has called off the cease-fire some weeks ago... which means you can travel safe enough, but just so you know it..

check out Soriano's Euskalduna Palace - Quick, I'm in Bilbao

Baque country....my favorite. First, don't be shy trying as many pintxos as your little stomach handle. The area is notorious for some of the best tente en pies.

In San Sebastian, I highly recommend checking out the Eduardo Chillida museum. Check out Rafael Moneo's Centro Kursaal. Make sure you walk on the boardwalk near Playa de Ondarreta to check out the Chillida sculptures built into the rocks.

It's a fair assesment that if you are going to Bilboa, you are going to see the Guggenheim so I won't reiterate that. I'd check out the Museo de Bellas Artes. They have a fairly extensive collection of Basque painters, and a great permanent collection (Picasso, Francis Bacon, Goya, El Greco).

I kinda thought the Bellas Artes was a waste of time.

In Madrid: make sure you go to the Rastro (market) in Lavapies on Sunday mornings (if you are there on a Sunday). The Reina Sofia is cool, and free on Sundays. Definitely spend some time in the Retiro (major park in Madrid) and avoid the Sol area, it's packed with tourists.

In Donosti (San Sebastian), go to the Peine del Viento and see the Chillida sculptures, also take a stroll along the Boulevard and if you have a car go to Mount Igeldo. You'll probably run into a Jatetxea (restaurant) up there which is probably just a kitchen with some extra chairs, and you can walk around and look at the cows and wildflowers too.

theres a pinchos restaurant called la cuchara d san tomin i think
its in the old quarter of san sebastian in the northernmost part of the quarter. its pretty damn good. they have a seared foie gras one thats super tasty.

the chillida museum isnt bad either.

and the old quarter of bilbao has a pretty interesting nightlife
if youre up for a beer. there are a ton a tiny hole in the wall bars w 'musica heavy'... more or less heavy metal. its the most bizarre thing ive seen in spain in way of cultural phenomenons. maybe youll get lucky and see some drunken locals pushing a broken down wheelbarrow down an alley pretending it was a bull a la san fermines

you can also take a day trip up to the basque area of france. its pretty nice.

you could go see some campo baeza

San Sebastian is one of my fav cities of Spain....

but i will go up to Biarritz (the french Basque Country)to have some nice surf and scene

and also depending on what time of summer ur at, check out the Hendaye and other small towns (around Biarritz), for their local parties...they get wild, lot of vino, yummy!!