Last June I spent a couple nights here and it was the best hostel I've ever been to:
http://www.chab.be/welcome.asp?l=en
There's a really nice atrium type lounge with pool table and bar, ourdoor courtyard for when its nice....
I went there alone and am still in contact with llike 5 of the people I met just randomly at the hostel lounge! There's a bar that opens at 5:30 with 1 euro beers, good place to make friends before hitting the town.
AMAZING gardens on the walk to the downtown area, everything's a short walk.
Definitely spend time at this one