A number of interesting sites can be visited all around :

the ornithologic reserve of Marquenterre the estuary of the river Somme (Le Crotoy, Saint-Valery, le Hourdel, …) the Abbay of Valloires and its gardens the Abbay and the village of Saint Riquier, (welcoming in mid-july a famous music festival). the old town of Montreuil sur Mer and its walls.

 

Boulogne sur mer, its old historical upper-town and the Nausicaa oceanographic
center.
Le Touquet and the gorgeous beaches of Berck/mer,Fort Mahon and Quend-Plage.
You can usefully refer to the guide vert Michelin Picardie-Flandres-Artois or visit the website of the Departement Comitee for Tourism http://www.pas-de-calais.com
or http://www.opalenews.com.