Hi Simon_h
Leaving aside excursions and the excellent ADAC maps, for one moment.
I got bored at work, so knocked something up in a few minutes. It is not the 'best' route, but it gives you an example why it is difficult to plot routes for other people.
You want to go from Calais to Contern in a day and then back again in a day. The purpose of your jaunt is therefore your arrival in Contern (to do whatever it is they do in Contern) and then your return home, all we hope in one piece. It's that purpose that you have to put to the forefront of your mind.
A very direct route shows, 266 miles in about four and a quarter hours. An alternative very quickly hacked out route, leaving the motorway at Cambrai, then heading roughly across country, is 277 miles (only 11 miles further) but takes closer to eight hours (twice as long). Ignore the small glitches, it was just me being lazy and dragging the route around.
Look at it another way, perhaps? What time do you want to be in Contern? If you leave the train and hit the road at about 10:20 Frog time, the fastest direct route will get you to Contern (no stops at all, mind) by about 14:30. The alternative and just eleven miles longer route, will have you arriving (no stops at all) by about 18:30. A considerable difference, I think you might agree and useless if you need to be in Contern at 15:30. Of course the times are approximate, we have no idea how fast you ride, how often you want to stop (or why) or anything much at all really. Similarly, none of us have any idea if you think two hundred and fifty plus miles (in the possibly peeing down rain or blasting sun) is an epic day's journey. None of this is to say we cannot come up with something else, but you need to give us a few more clues, please
One idea, might be to cut across: Cambrai, Le Cateau, Hirson, Charleville-Mezieres, Sedan, Virton, Longwy, then under the bottom of Luxembourg city to Contern. This is one of the classically simple ways to Luxembourg and may well fit your bill. I think it is the way I would go if I needed to go to Contern, in a day and wanted to avoid too much motorway. Indeed, I have used it several times, just to go to Luxembourg, as it is easy. I have called the via Hirson route alt2. As you will see it is about 257 miles in about seven hours. Just ten miles further than the basic quickest route, but three hours longer. But, just ten miles less than the longest route, more importantly up to an hour quicker.
So what have we learned?
(a) Plotting someone's routes is not easy. Not least, as nobody but one person alone knows what they want / need / can or cannot do.
(b) There is a huge difference (not necessarily in mileage, but in time) between taking motorway and non-motorway routes.
(c) The simple dropping of smaller 'scenic' roads in favour of more direct 'National' type roads may make a small mileage difference but a significant time saving.
(d) Stopping off to gawp at sites, have lunch, tea, coffee, fuel up, donning of waterproofs and taking off gloves will all add to time taken, not detract from it.
Cheers,
Richard
Unless you get seasick, in which case you may see your breakfast twice. 
