which of these routes 5 nights cycling through Belgium?

I've cycled extensively around Leuven, but the terrain is anything but flat. The hills aren't nearly as high as in the Ardennes, but for short portions, they can be very steep. If you decide to ride around Leuven, head south through the Heverleeboos towards the village of Vaalbeek, and from there, go to Zoet Water (follow the signs). There's a small lake here lined with several good restaurants.

Some parts of East Flanders also have some short but very steep hills, although I haven't personally ridden in this area.

Limburg province gets hardly any foreign visitors, and although it lacks world-beating sites, it's overall a very pleasent place for cycling. The towns aren't overly scenic, but are consistently agreeable. South of Hasselt the scenery is a little better, but can be a little hilly. North of the city, the terrain is mostly flat, but towns are far more sparsely spaced.

Thanks very much - great advice - so helpful!

Flatter is better given my 8 year old will mostly be towed by me (we use a Followme Tandem Coupling, Swiss product). What's you opinion of West Flanders as a cycling destination? From what I can figure out from afar it might not be as charming as areas further East ....

I have tried to put together this about 270km long bike itinerary. Especially the southren part of the route is not completely flat but with Google Maps you can get an impression if it is what you expect about your coming trip.

From Lille you can follow the D941 / N7 to Tournai (Doornik in Flemish). Lovely historic market square and that magnificent cathedral, but still undergoing renovation works I geuss since my visit two years ago. Worth a visit too is Musée des Beaux-arts, however likely not so interesting for young kids. On the way to Mons / Bergen you can visit the car museum in Leuze-en-Hainaut, Château de Belœil and theme park and zoo Pairi-Daiza.

Mons is a lovely small town with a nice market square and worth to visit is the church of Sainte Waudru. Inside there is the Car d’Or, the carriage playing a central role during Doudou, the procession held every year in May. Further you can climb located in a nice park the belfry for a panoramic view over the city. Nowadays the train station there is the construction site of an over the top project of Spanish architect Calatrava regarding the size of Mons.

East from Mons you can bike to Strépy-Thieu for the new ship lift and Canal du Centre for the historic UNESCO listed ship lifts. If of interest you can cycle along the sloping lock of Ronquières too on the way to Waterloo for visiting the battle fields of 1815 there.

Next Leuven, famous for its richly ornated town hall and not to mis is the beguinage “Groot Begijnhof” and the Abbey just outside the town centre there. On the way to Mechelen there is another well known zoo in Belgium Planckendael. In Mechelen you can climb the tower of the Rombout cathedral for a panoramic view and brewery Het Anker has tours. Not to mis to my opinion is Kazerne Dossin, Belgiums most important Holocaust museum.

Next place is Lier before finally arriving in Antwerp with a route through Zurenborg, well known for it’s turn of the century architecture. Antwerp has a well known zoo too, one of Europes oldest and still with some buildings dating back to the 19th century.

The thing about cycling out in West Flanders is it can be windy . . .

I'd rather tackle a a few hills than deal with wind all day in open flat country. :-)

You might look at adding Hasselt and the Bokrijk open air museum. Leuven is really a neat city, as is Ghent.

Have fun!