Up to De Panne on the Belgian Border. Lovely little town with a pristine beach and town centre.
After that, 20-30 mins away, Ypres, which is a bit of a tourist hot spot, but still a pretty
good place to visit. Come back via Cassel, on the largest hill in the area, and read about its involvment in both World Wars.

Go see that gigantic town hall type place just to marvel at the quite ridiculous number of windows it has.

Alternatively the Baie de Somme isn’t too far away. Le Crotoy whilst very touristy has a fantastic beach and lots of cafes and restaurants.

If you are taking in WW2 sites then la coupole is pretty decent, as is the blockhaus just up the road.

Biggest aquarium in Europe is at Boulogne-sur-mer.

Visit the battlefield of Crécy 1346. Spoiler alert: France 0, England and Wales 1 or more accurately arrogant toffs in shiny armour on horses 0 peasants with longbows 1

Le Touquet would be my choice although the aquarium is a good shout too.

One of the nicest legs of one of my bike tours was Calais to Crotoy – WW2 bunkers, Napoleans monument, Boulogne was nice, Etaples big casaulty evacuation point for injured WW1, le touquet – etc or you could head in land and do Azincourt – nice little museum and you can walk around he battlefield 1415 as above french nobility 0 longbow 12