Wonderful Park for Athletes
The Canal de Isabel II Park located right off Santa Engrancia Street between the metro stops of Rios Riosas and Cuatro Caminos (light blue line) is a little athletic oasis in the middle of an otherwise urban neighborhood. There are tennis courts, soccer fields, a driving course for golfers, and the whole installation is circled by a running path.
My favorite part (let's face it, I'm no athlete) are the boardwalk and fountains, surrounded by rose gardens and Mediterranean trees. It's a great place to throw out your towel, have a cold drink, and try to imagine you're at the beach, not in the center of the city!