Looking at May 2011 for a trip to Spain. Considering 3 nights Madrid and 4 nights in San Sebastian (which I have heard a great deal about lately). Thoughts on this itinerary and things to do.

Is this your first trip to Spain?
3 nights is a little short for Madrid particularly if one day is going to be jetlagged and you'll probably also want to do daytrips to Toledo and Segovia.
What are your interests?

Sounds like a great plan. In short, Madrid is museums, history, theaters, music and the best nightlife, while San Sebastián is food heaven, beautiful landscapes, long walks, some of the best city beaches in Europe and my favourite city on earth.

Some Madrid suggestions:

Have breakfast at Café del Círculo de Bellas Artes in down town Calle Alcalá, 42. One of the city's most emblematic cafés. Grab a window table and watch Madrid wake up on bustling Calle Alcalá. Later in the day, you should take the lift up to the roof topp terrace (the azotea) for the most spectacular views of the city. http://www.esmadrid.com/en/cargarApl...ntificador=179
pictures/videos: http://11870.com/pro/circulo-bellas-artes

The Reina Sofia contemporary art museum. The second floor with Picasso, Dalí, lots of other painitings, posters, films, photos etc. completely blew my mind off last year.
http://www.spanish-fiestas.com/madri...fia-museum.htm

(And of course Prado and the Thyssen-Bornemisza museums)

A stroll in beautiful Retiro Park. http://www.aviewoncities.com/madrid/parquedelretiro.htm

Sunday shopping at the Rastro market: http://www.madrid-guide-spain.com/el-rastro.html

Watch the sunset over the Guadarrama mountains from the terrace at El Ventorillo in the Vistillas park. Map, video and some pictures: http://11870.com/pro/restaurante-ventorrillo

Have lunch or some tapas at Casa Granada, on the 6th floor of an apartment building just off Plaza Tirso de Molina. Hard to find and a great terrace. http://www.tripfilms.com/Travel_Vide...ada-Video.html

Medina Mayrit, an Arab bath-house just off Plaza Mayor in Calle Atotcha, 14. (Mayrit is the old Arab name of Madrid, meaning running water and reffering to the water sources beneath the city). http://www.medinamayrit.com/

Café Central, just off bustling Plaza Santa Ana, is a fabulous jazz venue with live performances from the best artists every night at 10. Entrance 11€.
http://www.tripadvisor.com/Attractio...al-Madrid.html

Experience world class flamenco at one of the best tablaos. Madrid is arguable the flamenco capital of the world. Although the art form comes from the South (Andalucía), everybody has to conquer Madrid to get to the top. On their way there, or as an opportunity to try out new things and get an up close audience, many of them work the top tablaos. I highly recommend Casa Patas and Cardamomo, both close to Plaza Santa Ana.
http://www.casapatas.com/
http://www.cardamomo.es/

Prepare yourself for the San Sebastián food experience at Txirimiri in Calle del Humilladero, 6. In the midst of one of the best tapas/restaurant districts in town, in and around Calle Cava Baja in La Latina.
http://www.tripadvisor.com/Restauran...ri-Madrid.html

I never leave Madrid without having had the squid in its own ink and the fabolous cod at Casa Revuelta, also in the same district.
http://11870.com/pro/casa-revuelta

You should also try the excellent salmorejo, a somewhat thicker variant of the tomato/vegetable soup gazpacho, in Según Emma just behind the San Miguel food market. http://11870.com/pro/segun-emma

Fine dining at El Mentidero de la Villa:
http://www.mentiderodelavilla.es/
http://11870.com/pro/el-mentidero-la-villa

This site is great about Madrid. http://www.esmadrid.com/en/portal.do

The article "A different kind of Spain" gives you an idea of San Sebastián and the region: http://marshlands.blogstream.com/

You can do it. It would be a great trip if you had 10 days. You will spend almost two days traveling from Madrid to San Sebastian and back but they're both great cities.

ttt

I think it sounds fine and would skip day trips from Madrid but do drive to Bayonne for a day from San Sebastian. definately open jaw your flight though and go straight to San Sebastian and out of Madrid. Alternately you could just do a week in either place only or add on a couple days as stated above.