Day trips from Madrid & Barcelona?
 

How much of Spain is it possible to see using these two cities as bases of operations? I'd rather not check into a new hotel every couple of days, but I don't want to miss anything great. Thanks for your advice!

Barcelona is a very interesting city but Sevilla also is and you can make interesting day trips to Carmona, Cordoba, Ronda, Cadiz, Jerez and even Granada. Madrid is also worthwile and interesting day trips from there are for example: Segovia, Toledo, Avila, Cuenca, La Granja, Chinchon and Aranjuez. Salamanca and Santiago de Compostela are great too, but a little further.

Madrid and Barcelona are great homes bases, but you will be missing something amazing if you miss Southern Spain (Andalucia). For a any Spain first-timer this should be a mandatory stop. I would suggest Seville as a home base, with day trips to Cordova, Pueblos Blancos (a car is a preferred mode of transportation), Merida, the coast. Granada is a bit far, but I have read of people who have day trip to Granada and how they did it (La Alhambra is a must). If this is you first time to Spain, I would suggest, again, Seville and Madrid (day trips to Segovia, Avila, Toledo, El Escorial, Salamanca, Alcala de Henares) as home bases. If renting a car, we found AVIS to offer very good deals and they have a counter right off the exit from the international arrivals at Barajas Airport.

From Barcelona, Gerona is a nice place to visit for an old town walk. Lot of history.

Many thanks for the advice. I have an invitation to stay with a friend in Barcelona, so that remains on the itinerary, but I'll definitely add Sevilla as the third anchor city for my visit. Thanks again and happy new year!