My wardrobe is in horrible need of an update, but this is still a no brainer - travel. A cruise might not be my first choice (as many others have mentioned), but it's still a hell of a nice way to get away from it all and not have to worry about scheduling anything. I get bored within a day or two of sitting on a beach somewhere, so the changing scenery of a cruise alleviates that somewhat. Still, you're young - go backpack across Europe for a few weeks, hit up somewhere traditional like London/Paris/Rome/Venice if you feel like playing it safe to start, or go with something more exotic - safari, SE asia, etc. The world is full of so many experiences that can't be purchased in stores, and souvenirs/memories can last longer than more material things anyway.
Nick
