As you are from Germany i will give you some links of a German Rail TV show to raise your travel lust for France and you even get maybe some Route suggestions :)

Côte Bleue Part 1

Côte Bleue Part 2

Tendabahn

Schluchten des Zentralmassivs

and to get from Netherlands to France you could travel with the Kusttram along the Belgian Coast :)

Küsttram

Le Mans is one of the most underrated historic French cities - its elevated core (cite Plantagenet) is almost completely unspoilt, and the half-timbered houses haven’t been converted into chain-stores as in some other places. The cathedral is fairly large, and took long enough to build that the early parts are Romanesque while the later parts are fully Gothic. Needless to say, Le Mans also has much of interest if motorsport is something that interests you. From Paris you can go by TGV, or you can save €10 and go by a direct TER which also calls at Versailles and Chartres on the way.

However, it’s not a small town or by the sea. One extremely fetching place that is both of those things, also connected by direct train from Paris, is Granville. For an interesting trip extension, you can go for a daytrip by ferry from Granville to the St. Helier in the Channel Islands (1h30m crossing), the only part of Normandy still under Ducal control.