The nightlife isn't that bad, of course it depends on what you are into. One of the main hangout spots for young people is the amphitheatre by the river, because...well drinking outside is cheap. There are plenty of other places you can go though. Vavilon for the more... how to put this, douchey kind of atmosphere? It's a typical Serbian nightclub. There are several beerhouses like Labeerint, more alternative spots like Sunset Bar or Feedback (the main venue for anything other than turbofolk music, from electronic to heavy metal). You could even give a shot to some kafana (what a pub is to a Brit, a kafana is to a Serb) for a more traditional atmosphere. I'd avoid Stara Srbija though. Good news is everywhere is mostly dirt cheap.

As for the transport- buses. This is Buslandia, buses go everywhere all the time, just check online. If you want to visit a nearby country, Sofia, Bulgaria is probably your vest bet, depending on how far you are willing to travel.

How'd you decide to go to Niš though, you skipped two bigger and closer cities?