I'd see what flights are like in and out of Schipol for that period. Otherwise can second Brussels, if you're into beer it's amazing and the food is good, too. It's more of an "experience the city" kind of place than "a million attracts to sightsee" kind of place.
Brugges is pretty but I wouldn't recommend a solo 24 year old guy stay there overnight. It's not really aimed at your demographic, if you know what I mean. Nice to walk around for a few hours though, and not far away from Brussels.
Utrecht is cool, like a smaller version of Amstersdam and more of a University town vibe. Also great beer bars.
I haven't visited but Antwerp is rumored to be sort of cutting edge hip these days.