Hi,
Regarding the WW I sites...some excellent suggestions made by Will above.
I'll comment on northern France: Are you interested in British, US, Canadian, German or French sites? Notre Dame de Lorette and Beaumont Hamel are pretty poignant, grim. There is also a museum nearby of the war in Notre Dame de Lorette. In northern France if you're motorised driving in the Somme area, north of Amiens en route to Arras, you'll see British and Commonweath cemeteries dotted around the countryside. The tall white cross and the green road signs indicate spot.
In Arras itself since the city was the site of the BEF General HQ there is a large British mil cemetery and memorial at the edge of town, takes a bit over an hour to walk it from the train station. It is located next to the French Army caserne, which I believe is not open to the public if you want to visit the French Army.
Spend a few hours or so walking around the centre ville in Arras, you'll come across plaques referring to the 1917 Cambrai tank operation and also the events of 1940 and 1944. From Arras you can get to Vimy, best is to go by rental car. How much time do you intend on spending here? In between Arras and Vimy is Neuville St Vlaast where another memorial and Museum on the war are located. I spent most of the afternoon the first time going through these places.
I was in Cambrai once in the centre ville, didn't see plaques specifically on WW I, maybe I missed them, other than general ones with "mort pour France"
Two other sites in the Somme are the towns of Peronne and Albert, the latter being HQ for the British. Albert has a WW I museum but the biggest one on the war in France is in Peronne. With a rental car they are both easily accessible., not that far from each other.