If you like that sort of thing there are some great tunnels at a little place called Vauquois, this was a heavily fought over hill where the two sides kept digging under each other and blowing up tons of explosives. Look it up in Google Maps, the site is just North of where Google shows the village, it shows the road to the car park and then trails - these are where you can walk around, in sattelite view you can clearly see the craters where the village once stood.
The village dissapeared in a series of huge craters, these are still very evident and there are a number of Trenches left pretty much as they were at the end of the War.
They sometimes open up the tunnels and run tours, I think last Sunday of each Month, the tunnels are also pretty much as they were left at the end of the War.
Well worth the trip, there is also a small museum, but if you rock up in the week this will most likely be closed, but you can walk around the trenches and see the tunnel entrances.
Just be aware the tunnels are badly lit, damp, cold, low in places and slippy. They have not been re-paved with perfect grippy tarmac, widened for the benefit of the fatter folk of the 21st century or raised the roof in recognition of the fact we are all a bit taller now and do not like stooping, and being in France health and safety have not come along and insitied on hand rails and brilliant lighting - which is why I like it.
You can visit this, and both the main forts (Vaux / Duoamont) in a day, and even take a few unpaved roads in whilst your at it.
