The pond at Retiro is a cool place to spend a day with your significant other, paddling a rowboat around, enjoying the sunshine, street performers, and monuments. You can rent the boats for pretty cheap (around 7 euros per hour, I believe) and hang out. The pond is full of people-friendly carp which gather and splash (begging for snacks!) whenever you get near the water's edge. Anyways, it's not the most exciting thing in Madrid but everyone needs a day at the park now and then.