For a great night on the town, walk down Av Arístides Villanueva, where it’s bar after bar; in summer entire blocks fill with tables and people enjoying the night. On the other side of town is the Alameda, which is grungier and less salubrious, though you might be able to catch some live music. Wine is available everywhere in Mendoza (right down to gas stations).
Finding a dance floor generally means abandoning downtown for one of two areas: the northwest suburb of El Challao, or Chacras de Coria, along the RP 82 in the southern outskirts. The former is reached by bus 115 from Av Sarmiento. Chacras de Coria, a better choice, is reached from the stop on La Rioja between Catamarca and Garibaldi by taking bus 10, interno 19, or from the corner of 25 de Mayo and Rivadavia by taking bus 10, interno 15. In both cases simply asking the driver for los boliches (the nightclubs) is enough to find the right stop.
The nightclubs in both El Challao and Chacras de Coria are all right next to each other, and you can walk along to take your pick from the ever-changing array. Most don't get going until very late, which presents problems getting back to hotels in the city.
Many visitors to Mendoza (and mendocinos for that matter) find the effort involved getting to these places far outweighs the fun they have there, often opting for the smaller bars along Av Arístides Villanueva and the Tajamar.