I live in central London, cross to France via Calais and go there reasonably often.
Depending on what time your crossing arrives / returns and quite how far you want to go on day one and / or return from on the last day, I would suggest one of the following:
A. A trip down to the Morvan, via roughly Verdun. Say three days in the Morvan / a ride across to the Côte d'Or for a day out and back to Calais. Two days to get there, three days there and a day (or take two) to come back.
B. A bit further would be to go to the Vosges, which are sort of the French side of the Black Forest. This you could do by going Calais, Verdun and then a longish day down to Colmar and the Vosges. Come back via the Morvan, perhaps?
C. A trip westwards into Normandy and Brittany, perhaps? But, if your wife thinks that looking at yet another German pillbox or another museum is reasonable grounds for divorce, you only have yourself to blame.
D. Not going to France at all, but heading across into the Belgian / Luxembourg / German Ardennes. Lots of really good riding and if it all goes to hell in a handcart, very quick to come home.
I wouldn't rule out motorways, if only to get from A to B quickly. Nobody will ever know and there are no public awards of medals for avoiding them. For instance, if you go down to the Vosges, you can go via Nancy. You'd be well advised to take the motorway (at least around the city) as it's a real grind getting through the place, trust me.
Similarly, it's near enough 350 miles from Dijon, about halfway down France, to Calais. You can do it in a day, missing out the motorway and still use some really good roads.
Do not let anyone tell you that it's all boring they (and not the roads) are just dull. But 350 miles is further than you want to go and / or you might not have enough time. So, do the last 100 miles back to Calais (say from St Quentin or Cambrai) on the motorway. It's peanuts. There, you've just done 350 miles in the time it might have taken you to ride 250.
I have posted several threads on suggestions, very similar to the one you are looking for ideas on, often along with GPS downloads of the routes to take. Alternatively, have a look at the RiDE magazine website. It really does have some very good suggestions (and pre-made routes in GPS and Google map formats) to answer your appeal for help.
http://www.ride.co.uk/magazine-routes/routes-from-ride-magazine
Richard
PS Do get some good maps. People do sneer at them in the 21st century but they will never let you down. I promise.