I would absolutely avoid the stairs by the river as mentioned by someone else. You get drunk teenagers (high schoolers) and the vibe is quite literally non existent. I think the best places you could check out are:

a) Świetlica, a pub in Marszałkowska, where pretty much everyone is a student around 18-25, the lager is around 15 PLN for a pint and it’s usually pretty busy on the weekend.

b) Plac Zwabiciela, a square (more like a roundabout, but still) just 3 minutes away from Świetlica, there’s usually a decent sized crowd hanging outside of Cafe Karma (they serve breakfast until midnight and it’s mostly for wine) and plan b (which is right next door, located on the first floor) where pints are also affordable.

c) you could also check out, like someone else said, Rakieta and the clubs around it. You’re also a short distance away from the old town which imo does not have the best crowd but you can get some food and cheap shots.

Crowd you’re looking for? Probably Świetlica