Like to add the follow:
About WOI in The Netherlands is of interest Huis Doorn east of Utrecht, where former emperor Wilhelm II lived in exile till his death. The place remained in the orginale state, absolutely worth a visit, enthousiast guides.
Overlooked and hardly known is the WOII Battle of the Scheldt in the Delta Region of The Netherlands. The loss of the logistical vital port of Antwerp and the Scheldt Estuary for the Germans was one of the main reasons that leaded to the Battle of the Bulge. However there are just little museums dedicated to it, so it’s more fore those really interested and looking for a comprehensive idea about this stage of the invasion by the Allies. This operation was done by the Canadian, British and Polish armies.
dgnagle4 - You mean Ostend (Oostende) where you can visit the Atlantik Wall, lots of tunnels, trenches and completely fit up bunkers. As far as I know there are no more passenger ferries between Ostend and the UK. There is one between nearby Zeebrugge and Hull, but expensive. Like Mark G stated is Calais the place for crossings to the UK.
Ironical, now with the 100 year commemoration the WOI section of the Royal Museum of Military History in Brussels is closed for renovation! Hard to believe, but it’s a fact. However there is a very interesting ’14-’18 exhibition, with an interesting explanation about the lead up to this conflict from a general and more specific Belgian point of view. The unique material of the permanent exhibition however is stored elsewhere during the period of renovation.
France - Like Mark already says, both La Coupole near St. Omer and Blockhaus near Éperlecques are highly to recommend as well as the superguns of Mimoyecques, where also inside a little memorial of Joseph Kennedy, JFK’s elderly brother. More west Musée du Mur de l’Atlantique / Batterie Todt with unique K5 railway gun.
Dunkerque for museum about Operation Dynamo. Nearby Gravelines, one of the fortified places by Vauban.
U-boat bases along the French Atlantic coast like the one of Saint-Nazaire.
Fortifications of the Maginot Line like Fort Hackenberg.
Musée de la Grande Guerre in Meaux near Paris. Also absolutely worth to visit, the visit begins with the explanation of the lead up to the conflict, here more specificaly the political, military and economical tensions between France and Germany since 1870. Musée de l’Armee in Paris.
Not to forget Verdun. West of Verdun, the Meuse-Argonne American Cemetry