Amsterdam: Flying Pig Downtown - great people, great party, great place all around.
Brussels: Sleep Well. This place is basically brand new, nice, and quite safe.
In Amsterdam a cafe is always a great place to "shop"
Just ask around the hostel for other stuff--someone will know about something cool.
In Brussels Grand Place is the hot spot for coolness. I personally did not like Brussels but others have. The waffles are awesome and its a pretty modern city now. Sleep Well hostel is right next to a huge shopping mall. Hit up some guide books and others at the hostel.
I hope this helps!