Based on your requirements, you've got pretty much 4 whole régions to select from. Probably easiest to tackle them in turn!
The best one for your family coming in from Rhine-Main is probably the Champagne-Ardenne region. City wise, try Reims. It'll be about a 3 hour drive for you to Le Havre afterwards, but you'll get to see a lovely old town, the cathedral, and you can taste Champagne! Also the option to meet in one of the villages outside Reims, if you'd rather countryside + Champagne tasting from one of the producers.
Not that different in travel time is the often overlooked Nord Pas-De Calais. Should be similar from Germany, and 3 hours to Le Havre again. The city of Lille is very nice, with some great art (modern and classic), an old town, and a citadelle you can walk around the parks surrounding it, plus there's a zoo!
Easier for you, but slightly further for them is Picardie. You've already mentioned the towns of Amiens and Saint-Quentin, but I'd suggest you follow Annoyed's advice and go to the nearby Château de Pierrefonds. If you've seen the BBC series Merlin, it's that castle! Maybe 2.5 hours to Le Havre, as it's slower roads.
Finally, good for you, less for them is Haute-Normandie. The town of Rouen is very nice, and very handy for Le Havre!
Oh, and while Le Havre itself isn't as nice as somewhere like St Malo, it could be worth a brief stop on the way to the ferry!