the Ujezd is a great place for beers and to meet some genuine czech people, definitly check that out. if you plan on staying at the elf hostel they know of everything going on.
The Blind Eye-- good bar to smoke and meet some ex-pats
basically if you get a map of the whole zizkov region your set. try to stay away from the old town, although fun, way more expensive than anywhere else in the city. definitly try and meet all the natives you can because they know how to party and where the party is at. But definitly check out those two places i told you. you wont be disappointed.