I did Brussels in 6 hours once, on a day trip from Antwerp when my partner was living there, about 10 years ago so things have probably changed a bit. Atomium, Mannikinn Pis, Grand Place, Tintin musueum, Victor Horta museum (art nouveau central), and the iguanadons in the National Museum are what I remember, but I think that there were more attractions there too.

The trains, while cheap and frequent, are actually quite slow by UK intercity standards, though fast by West Riding ones. Unless they've finally put the TGV in. For an afternoon in Antwerp: check out the railway station bar, take a trip up the KB tower to see the place, and see if Extra Large is still open on Oude Beurs. If it is, buy a giant Zippo lighter.

Drink De Koninck <- sound general advice. Duvel, Kwark, Leffe and co are all very well, if you want to lose the ability to walk and/or talk about 2 hours sooner than you were expecting to.