Madrid is indeed very nice. Perfect destination in early spring or late autumn as winters are very short there. Avoid in the summer as it gets stupidly hot there. One reason I like it is that it's far away from the beaches and package tourists. It's huge modern city. And there's plenty to see. I love the public parks there. There's a huge new park on top of the inner ring road which they partially covered up. Perfect place to hang out on a warm day. Also did wonders for the nearby neighborhood which are now quiet and a lot less smelly than they used to be.

There are a couple of other museums well worth visiting near El Prado. El Prado can get very busy because it's on everybody's list of things to visit. I've been there on a quiet day at some point and it's very enjoyable. But when you have to queue up for 45 minutes just to get in, it's probably a lot less nice. Thyssen-Bornemisza Museum has a pretty amazing collection and is right across the street. And down the street is the Reina Sofia, which has a nice modern art collection (think lots of Miro, Picasso, Dali, etc.).

If you have time and a car, driving around Spain is very enjoyable. I've seen most of it's larger and smaller cities over the years.