I did a day trip to Brugge from Brussels back in March. The train was easy (about an hour or so if I remember correctly) and inexpensive. The walk to the town from the train station takes a while, but isn't too bad. The town itself is very fun to walk around - lots to see.

If you are the type that like to explore things more in-depth, I would recommend spending two days there.

As always, be alert and attentive on the train platforms and in touristy areas. Pickpocketing is common. I had my work cell phone taken by two guys working as a team on the subway. I let my guard down for about a minute and they were that fast.