Hi, Jules! My husband & I just returned from a week's vacation in Bruges.
The train system is very efficient in Belgium, albeit a bit expensive. But, if you buy the duo ticket ( if it's in season that you'll be visiting Belgium -- someone said it may be a seasonal thing?), you can make it work.
To give you an idea, from Bruges we went to the Herge museum (Louvain-le-Neuve), which is some ways southeast of Brussels, and it cost us about 63 euros round trip on first class.
We decided to go with first class because we were always guaranteed a seat, and it's less crowded. We had bought a 2nd class ticket from the airport and couldn't find any seats on the train.
Also, you can buy the tickets on the day of. As for an open-ended return, I did not see that option.
Also, if you'd like to save a bit of money, there is a free shuttle from Bruges Station to the Markt area. They make various stops throughout the neighborhood surrounding the square. The free shuttle bus is at the Kiss & Ride. It's very non-descript sign. It's adjacent to where all the public buses come around. There's a little sign that says CENTRUM (or something like that). And the bus is a cute little box-like thing. DM me if you're interested in knowing more.