I always recommend Boulogne for day trippers who do not want to go far from Calais/Coquelles, and this was the area my love to France, well, Northern France was born.
I've been to a few places now and I don't think I have yet to find a favourite, they all are different. I enjoyed walking around the ramparts of Montreuil and Berques, and you could get a sense of history of how it was. I like the aquarium, harbour area and old town of Boulogne. I enjoyed the little park area of Hazebrocke, and the kite festival of Berck Sur Mer. St Omer and it's rocket museums are definetely of interest. I enjoyed Dunkirk, and the obvious history of the place. Obviously I've really summed these places up in short sentences :D
Le Touquet, called Little Paris, is definetely different altogether, enjoyed my visit there.
So over to you :)