Vimy Ridge is worth a visit, Cod. That's in Northern France. It was the scene of a huge battle (mostly fought by the Canadians as the Brits were all re-forming at the time). It has an enormous memorial opened by George V. It was the site of the first electrically lit tunnel ... you can visit this, and you can walk the trenches and see just how close the Allied/German lines were.
Profoundly sad .. but you don't need me to tell you that.
Poperinghe, very close to Ypres, is where they have kept the poles that the 'shot at dawn' men were tied to. It's also the site of Talbot House ... I have a thread going on about that at the moment.
There are a couple of nice places in Ypres itself, but generally speaking, all the pensions in Belgium and Northern France are to a very high standard.
Arras is very beautiful, and central to many of the sites.