Parque Retiro and a perfect day in Madrid.
You have to start it like this on the first day you arrive because any day after that you will wake up in the afternoon because you were out until 7 AM. In my opinion Madrid is not as much about where you do it, but how you do it.
Have a nice lunch in Malasaña. If the weather is nice, eat on a terraza in Plaza Dos de Mayo.
Take the metro to Parque Retiro and walk your lunch off.
Take a nap.
Meet friends for beers. Maybe back at Dos de Mayo.
Go to dinner at 10. Have some nice tapas and wine.
Go out for copas at midnight. My preference is for gin tonics.
Go dancing at 2. Stay awake somehow until 7 and pass out.