Margarita seems like a hung out place both for locals and tourists. It has good food and drinks including famous Argentinian Quilmes.
They have great onion pizza which costs about 8 pesos. A 1l bottle of Quilmes or Brahma costs 6 pesos.
Margarita is the standard meeting point by night for the young people and even for all those not so young. Fun ambiance, very good music and drinks, perhaps you wish eat some dishes at the evening, the atmosphera become more special with the advance of the night, some special espectacles every wednesday, on week ends until the sun does appear on the line of the horizon. Nice place, nice people.
Dress Code: Casual
Although Puerto Madryn boasts its own casino (with showgirls according to the adverts - I never visited) the truth is it's a sleepy town. There are several bars to hang out in but you might be better off enjoying a good meal and getting to bed early - most of the tours leave early in the morning because there's a lot of travel necessary to get anywhere - and you're unlikely to be at the port to party.
Most of the nightlife is based along Avenida J. A. Roca by the beach and everything is in walking distance.