In reply to FranC:
The Somme battlefields (about 2 hours from Calais) - interesting places to visit include the Thiepval Memorial, The Newfoundland Memorial park where there are preserved trenches, the Lochnagar Crater one of the biggest man made craters from the First World War, the Sheffield Memorial park to the Chums Battalions, and lots, lots more, including hundreds of War Cemeteries - the British ones are laid out as beautiful gardens. A great walking and cycling area generally quiet roads and farm tracks reminiscent of the South Downs in places
Recommended book "Walking The Somme" by Paul Read ISBN 0 85052 567 5
Nearer to Calais there is Vimy Ridge (about 1 hour from Calais) with it's magnificent and poignant Canadian War Memorial, and trenches and tunnels a short distance off the Calais motorway.