I've just come back from two weeks in Flanders. I'll have a more detailed report (or four) to post later, but right now I just want to recommend Kortrijk as a good base for a visit. It's a pretty, compact little town. You won't hear much English there, unless it's directed towards you. While there are tourists, most are Flemish themselves. It has a couple of nice museums, churches worth looking at, and a historic City Hall. It's also a great place just to walk around in, or to sit and have a beer on the market square. While I was staying there, they were in the midst of a town-wide event on the the theme of "play." Among other things, there was a fun fair, a pop-up playground (with a Wild West theme) in the market plaza, and you could jump out a window of one of the medieval towers--landing on large air bags.
Kortrijk has good rail connections. You can use it as a base for day trips to Ypres, Bruges, Ghent, or smaller towns in Flanders. Ostend, Antwerp, and Brussels would be longer rides, but doable.