First out, go across Belgium, to Mons for the first and last British cemeteries from the 1914 battle and the final advance in 1918, and to the forts around Liege and Namur.
Then start from the sea at Nieupoort, have a look at the sector the Belgians themselves defended, Ieper/Ypres for the Menin Gate and the sites around the salient, Tyne Cot at Passchendaele/Passendale, Langemarck for the big German cemetery there, Messines ridge and Kemmel, Vimy Ridge for the Canadian memorial, Arras and its caves, the many sites on the Somme between Albert and Bapaume, especially the Newfoundland Park, the Ulster memorial and Thiepval. Then carry on south into the French sector, the Chemin des Dames and Verdun in particular.
If you have a bit more time to spare and want to get some miles in, head diametrically across Switzerland to the Dolomites, visit the sites at Passo Tonale and on the Marmolada glacier where the Italians and Austrians fought in the high mountains, and down to the Asiago plateau (where the British also fought) and across to the Piave and the Soça/Isonzo in Slovenia.
Then think about another trip for the Eastern front, Romania, Thessaloniki and Gallipoli...