Plazas are awesome and I don't know why we don't have more in the U.S. (but we should). Check out this plaza near Puerta del Sol for drinks or even dinner. It's one of the prettiest in Madrid and has great bars and restaurants in every direction. Another option for the evening is to make like the locals and wait for the beer vendors to come around carrying plastic bags and grab a beer for 1 euro then drink in packs down the side streets. The plaza is also a good spot to meet before heading down the bar filled street, calle huertas.