Boulogne-sur-Mer is France's busiest fishing port and, for many British visitors, the first taste of France. Most people roll off the ferry and drive straight south — which is a mistake. The Haute Ville perched on the hill is genuinely lovely: medieval ramparts you can walk in their entirety, the Basilique Notre-Dame with its enormous crypt, and cobbled streets that feel nothing like a tourist trap. Give it a night, not just a lunch stop.
The lower town along the harbour has good seafood restaurants and a lively fish market. The Nausicaá sea centre is one of Europe's best aquariums, worth a couple of hours even if you're not travelling with children. And 30 kilometres north, Cap Gris-Nez gives you the most dramatic Channel view in France, with Dover's white cliffs on the horizon on clear days.