Reina Sofia museum is always a good idea. And it's never enough of Picasso's Guernica as well as the other modern art gems. Save some hours for visiting and then look round the corner and have some wine at NuBel.
After long hours at Reina Sofia museum time for going to Outer Space. It's called NuBel and it makes you wonderfully dizzy. Or, better, go there just before the museum - drink some wine and go to admire Guernica in 3D.