Can’t think of too many WWII sites in Belgium – although there is an Atlantic Wall site near Ostend which I very much want to visit.
Andyfla has nailed the sites around Ypres, plus there’s the Essex Farm site where John McCrae served, the German cemetery at Langemark, the cemetery at Poelcapelle where the youngest known soldier who was killed is buried, the grave of Noel Chavasse VC and bar at Brandhoek Military Cemetery, Talbot House in Poperinge (another lovely town).
The Last Post at the Menin Gate is always busy and special anyway – hard not to be deeply moved however many people are there (and it’s good that so many people still go).
Den Anker and Oude Tom are great places to eat in Ypres, and the Ariane and the Ambrosia are great places to stay. The Ariane is more expensive than the Ambrosia but both are excellent for their respective price brackets.
Haven’t cycled around Ypres but there appear to be dedicated cycle routes around WWI which go past some of the sites above.
Love Belgium – I’d live there like a shot.