Due to its location close to heavy industry, a large freight port and a nuclear Power Station, it would be easy to overlook pretty Gravelines (Grevelingen), but it really is worth making at least a day visit.
The old town is based around one of Vauban's famous star shaped forts which is still pretty much intact. Walking the walls of the fort is well worth the effort, and by the nature of things, you are never far away from your starting point.
A longer walk would be to follow the river 'Aa' as it flows towards the North Sea at Petit Fort Philippe.