Just to up the ante a bit, here's the route for the DIY "Four Countries 600" I did last year with a couple of mates:
https://ridewithgps.com/routes/19604413The idea behind it was to visit four different countries in one weekend. It starts in Dunkerque, France, then heads through Belgium, into the southeast corner of Netherlands and across the border into Aachen, Germany, before turning round and heading home again.
The original plan was to get the ferry over to Dunkerque and start from the port but that proved problematic so we ended up using the Eurotunnel. Public car parks in Dunkerque are free at the weekend so I left the car in the one near the station. There's a newsagent in the station that is open early and serves coffee and pastries, ideal for a light pre-ride breakfast, but we stopped in Bruges for a proper breakfast.
Much of the outward leg is along the canals and is therefore obviously very flat, but it starts to get a bit more choppy after 200km, and the bit between Aachen and Liege is actually rather lumpy.
Maastricht to Aachen is mostly along a main road but there's a decent cycle path.
After the turning point in Aachen, it heads back via the Drielaandepunt, the point where the borders of Belgium, Netherlands and Germany meet, and the highest point in the Netherlands. However, it was dark by the time we got there so we didn't see much of the view. There's about 2km of offroad trail up to the Drielaandepunt but nothing that isn't manageable on a road bike.
The route is designed to skirt around Liege but we took a slight detour for a sleep stop at the Formule 1 on the outskirts of town.
The return leg is again mostly flat and has lots of canalside cycle paths, but for those who like climbing, it goes through Geraardsbergen with the option to take in the 'iconic' Kaapelmuur. You could also take in some of the famous hellingen around Oudenaarde without going too far off route. And there's an obligatory section of pavé on the outskirts of Oudeanaarde as well.
Although the route was largely cobbled together from Google Maps, I was very pleased with how it turned out - except for several sections of cycle paths and roads that were being dug up, requiring detours. We seemed to have visited on Belgium's National Roadworks Weekend. Anyway, there were very few sections of road that I would choose to avoid if doing the route again - and I definitely do want to do it again some time.