i don't think you'll have any problems finding things to do!
for instance, you could sit on a bench and just watch and in a few minutes you can find a hundred things to check out...
one cool thing to look for in way of art is the traveling expositions of art that make their way around europe, in particular a series that caja de madrid (big bank) offers from time to time... the last one i saw was called "the road to the abstract," featuring the works of piet mondrian and vasiliy kandinsky. what a great show of works that was! i found out about that one standing in line at a caja de madrid branch speaking to a beautiful spanish woman (you should have been there, MM
).
of course, be sure to see the reina sofia and the prado (as well as the thiessen (sp?).
architecture is also one of my favourites... you have only to walk around the city for this...
by the prado there is a street that has a ton of booksellers selling books on anything you could possibly want to know. i found a book there that had all the street names of madrid and what they are named after.... cool stuff!
you will not have any problems finding things of interest in spain! have fun!