Where should i visit?

Planning to go on vacation during my next winter break from school, looking to backpack a bit through some new places for me. I have a decen size budget of around 4000, including travel. Yes, I'm looking to do the hostel thing, and I'm going with my girlfriend, who has a similar budget. We've been planning to do europe, start in italy, be in rome for the befana, then make our way through france, into spain. We leave from new york and plan to be gone for 21~25 days. But recently, I've been more drawn to visiting somewhere in asia, perhaps, tokyo or seoul, or maybe bangkok or hongkong. After working and going to school nonstop, wed like to take a trip that would be a once in a lifetime experience, or at least something wed get a lot from. Which should we do? Which do u reccommend and why? Thanks :)