Most efficient is taking the Eurostar to Lille and pick up the car there. Both Calais and Dunkirk are not really of interest, unless there is a specific reason. Dunkirk is best known for the blockbuster or also as Operation Dynamo. Close to the beach is a little museum and a monument commemorating the evacuation. But damaged heavily during WW2, but if you are looking for that both cities miss the charm to make them worth visiting to my opinion.
Further WW2 related places of interest are La Coupole and Le Blockhaus, two former V2 launch bases near Saint-Omer west of Lille. Closer to the coast are the four huge gun batteries near Audinghen, one of them is a museum. If WW1 is of interest Vimy Ridge north of Arras and the battle fields around Albert and mainly the Somme east of Amiens. You can also think about visiting Ypres / Ieper in Belgium too, not that far from Lille.
Lille is certainly a charming place to walk around. It’s a mix of French and Belgian architecture,the latter especially in Vieux-Lille with it’s cobbled streets. There is a huge museum of Fine Arts and an annex of the Louvre in Lens south of Lille.
Depending the route you want to follow both Arras and Amiens are worth a stop. The first for it’s unique two main squares and the latter for it’s famous cathedral and adjacent charming streets.
Driving along the coast:
Cap Blanc Nez for it’s magnificent views, you can see the White Cliffs of Dover if the weather is clear.
Boulogne-sur-Mer has still it’s medieval walls. The historical part of the town is not ouststanding, neverhteless worth a stop in case you are in the neighbourhood.
Le-Touquet-Paris-Plage, the seafront is ugly but found it charming around the casino. It’s a bit faded glory but the old fashioned villa’s give an idea how it once was.
Saint-Valerie-sur-Somme, lovely little sea resort at the Bay of the Somme, there is a steam train too.
The best part of the Atlantic coast north of the Seine is to my opinion around Étretat with tiny but very lovely places like Plage de Saint-Pierre-en-Port, Yport and Vaucottes. The coastal road between Fécamps and Étretat is very scenic. And if you like hiking, there are many stunning views along the trails on top of the cliffs.
Have to say this part of France with some exceptions is not the most outstanding, but with good planning, depending your interests a worthwhile itinerary is certainly possible. Places like Rouen, Bayeux and MSM are much discussed here, so I keep my contribution limited to what is to see north of the Seine.