Geeze! Lots of Philadelphia posters here on this board....have you been marketing your site in Philly Madridman?
Must see... in Madrid, I agree with the Royal Palace you can do it in a couple of hours. I also recommend just roaming the streets of old Madrid.. around Plaza Mayor, Puerta del Sol, Gran Vía, etc. The museums are absolutely wonderful to see, but if you just have one day, you might want to spend it soaking up the local flavor of the city and doing as the Madrileños.... tapeando and eating churros con chocolate and enjoying the holiday spirit on the streets.
I´ve been to Barcelona on a couple of occasions, but cannot recommend day trips outside the city for lack of experience. In the city be sure to visit Montjuic, La Sagrada Familia, Manzana de Discordia (The Block of Discord), Las Ramblas, Parc de Güell, and the Barrio Gótico. The Moll d´Espanya is Barcelona´s example of Penn´s Landing with a night life and several great restaurants... also worth a visit. I had eaten at rice restaurant there called L´Elxe (or L´Elche)... it was very good! Be sure to try the local cava (champagne) .. Frexinet, which is very available in the American market is from Barcelona.
Enjoy your trip!