This is one query I can definitely help with. I was over there at the end of April and stayed at Wimereux, just north of Boulogne. I rode up the coast to Cap Gris-nez and back, mostly on the quiet coast road, the D940, and it was superb. Great views across to the White Cliffs of Dover, and an interesting museum in one of the blockhouses of the Atlantic wall near Audinghen and lots of snack bars/friteries/cafes everywhere. Loved every minute of it.
We drove back to Calais the next day, and I made a mental note to do a day trip this way some time in the future. Near Escalles, the D940 has some lovely sweeping bends and a nice descent (Southwards) or climb (Northwards) that I really fancied the look of. Further South the road cuts through the sand dunes between Audreselles/Ambleteuse and Wimereux, with occasional views of the English coast all the way to Boulogne.
I think it is only about 35km from Calais to Boulogne, so it should be quite possible to easily get as far as Ambleteuse or Wimereux and back in a day. It will be worth the effort of picking your way across Calais from and back to the ferry port.
I have had quite a few daytrips over the Channel before, and quite like nipping across the Belgian border to La Panne and Ypres. Some of the French towns to the East and South (such as Cassel and Bergues) are quite nice too but, in my opinion, I wouldn't bother riding there by bike. Too many big roads to cross, Too many lorries using the minor roads as rat runs to the ports, and not enough scenery. (Very flat and quite industrial, with only slagheaps for hills). GO SOUTH WEST ON THE D940 COAST ROAD and you won't regret it.