Getting There
FERRY
DFDS brings you straight from Dover to French Flanders, and from my experience this is the most reliable service across the Channel.
From Dunkirk all of French Flanders is reachable within a 40 minute drive.
The Ferry to Calais is of course an option, but is sometimes prone to delays due to congestion and industrial action. You also can add another 30 mins onto your journey to French Flanders.
BYCYCLE
Arriving by bike is a great option as French Flanders is reasonably cycle friendly. The obvious route to use with your bike is the ferry from Dover to Dunkirk. Leaving the port in Dunkirk is a little unpleasant to start with but after a couple of kms you can pick up the quiet roads to travel further into French Flanders. The hills are mostly gentle, with a few more difficult climbs such as 'Mont Cassel' and 'Mont des Cats'.
SHUTTLE
The shuttle from Folkestone offers the quickest crossing, but you can then add 30 mins onto any destination in French Flanders as compared to arriving by ferry in Dunkirk.
EUROSTAR
This is a great option for those who prefer to travel without car or bike. Arriving at Lille Europe, there is a 5 minute walk to Lille Flanders from which there are direct connections to Dunkirk, Hazebrouck, Esquelbecq, Bergues and Bailleul.
To reach Cassel is a little more difficult, as Cassel station is a couple of kms from the town, and involves climbing Mont Cassel. This is fine in the summer with a backpack, but no fun in inclement weather or with heavy luggage. The solution is to take the train to Hazebrouck and then the bus line 127 on to Cassel (49 mins), which brings you direct to the Grand Place in Cassel, or pay about 25 euros for a Taxi.