I have cycled Calais - Ypres (not on to Gent - I caught a train onwards to where I was going). The route took me via Audriques, Rumingem, Watten, Cassel Ypres by back roads. It's more varied scenery than heading along the coast (frankly a bit dull) and has some climbing. Interesting sights on or near the route are Eperleques - has the V2 Rocket Bunker, Cassel is a pretty small town on top of a hill with some good restaurants, Ypres the WWI stuff.