Wow, that's a tall order Kieth! So many and so varied. Like a lot of people I usually run Dover, Dunkerque, France, Belgium, Luxenburg, Germany, Austria (Brenner pass), Italy, Venice. I have found interesting stuff in all these places but cannot easily give you advice cos it has usually happened when I got lost or was snowed in or looking for fuel or a nights lodging.
I always fancy detouring as you come down out of the Alps either in Austria or Italy, searching out some of those magnificent castles perched high on the mountain valley slopes. Or how about lake Garda just off the route (bet it's pretty expensive though. An easy one is to stop over in Venice, so much to see and feel.
Others will, I am sure give you better ideas of man made stuff to see, but trained as a biologist and geologist, I have no greater pleasure than just stopping and 'kicking over stones' in any number of deserted forests, marshlands, river valleys, shores etc. all along the route.