We will be in Bruges, Brussels and Antwerp at the beginning of June. Are there any attractions that we should consider buying tickets for it in advance?
No, not usually. The most popular attraction is the Belfry in Bruges, but in that case you’ll need to book a ticket one or two days in advance to be sure of getting your preferred time slot.
For a Bruges boat trip, you just can't buy tickets in advance.
PS:
BRUSK in Bruges is still unknown, as it will only open end this week, but I reckon it won’t be necessary to book in advance, certainly not more than a day beforehand.
MOMU in Antwerp (FashionMuseum) keep an eye on it with the Antwerp Fashion Festival 04-07 june
Bruges is busy all year; for Brussels you really never need to buy tickets in advance. There will be crowds around individual attractions (i.e Manneken Pis, for which you need no ticket) but buying in advance is convenience and not necessity. In Brussels I highly recommend the Royal Museums of Fine Arts - it is a complex of art museums. The Magritte and the Old Masters are very good.
Lavandula