When hungry in the Microcentro, the best idea is to head towards Avenida de Mayo or pedestrian Calle Lavalle and just take a stroll. Hotdog stands, fine dining, coffe shops, authentic Argentine restaurants, Asian food, pizza, North American fast food restaurants and anything you could want can all be found on this strip. However, many of the streets are a bit on the deserted side, making taxi options somewhat dicey and increasing the possibility of an unwanted encounter, so be careful.
It's not necessarily where the prettiest or the wealthiest go to play afterhours (that would be Palermo Soho), but there's a great time to be had in Microcentro, if you know where you're going. The bars are generally less showy than in other neighborhoods and the clientele tend to show up casually dressed and ready to get drunk and loud.
Le Bar and its more upscale ilk aside, the dining experience in the downtown part of Buenos Aires tends toward the kitschy, the cluttered and the colorful. Restaurants here boast more local flavor than the tourist traps of nearby Puerto Madero, less than the traditional Argentine parillas of the barrios immediately to the north, and they're not nearly as glamorous and modern as the international eateries popping up all over Palermo.
The nighttime restaurant scene tends to be noisy and crowded in contrast to the relatively deserted streets, but during weekday business hours, brave the packed narrow sidewalks of the Microcentro, and you'll find lower prices than in central BA's other increasingly inflation-addled neighborhoods and tastier food than the hole-in-the-wall trappings of some places might suggest.