Find the Raamstraat near the Leidseplein. It has a pretty awesome Belgian beer café who are absolutely well-stocked on a huge amount of great beers
Obviously aside from the obligatory walk through the Red Light District and all the musea, there's not really that much to do in Amsterdam. It's a bit of a shame that you're here this weekend and not the next, when it's Queen's Day and there's all kinds of stuff to do!