I am somewhat geographically challenged when it comes to Europe.
Are there any volcanic craters in Europe that do not have obvious volcanic activity during known history?
Basically I am looking for a magical location of a mountain with a hollow at the top, where there would normally be a pointy summit.
Anything in the French Massif?
The massive amounts of the massif central in France are exactly that, another would be the Laach lake /Lachersee near Koblenz in the Rhine.
Otherwise a 10 second search on wikipedia turns this non-exhaustive list of volcanic caldera lake sorting by continents.
drats!. I had searched on "volcanic craters", not "volcanic crater lakes".
humm,... none of them seem to have a distinctive mountain cone. Lac Pavin seems to have noticeable crater walls. It may have to do.
bonus - distinctive local cheese.
depending on your taste you might also decide that in your group's mythic europe, your favorite crater lake has crater walls despite what the real world equivalent lake has.
I'm not an expert, but I suspect that if you look up the mountains of the eiffel region in western Germany, you might find other such example.
Also, if you want a crater with distinct walls, might I suggest Kaali, it's not volcanic but from a meteoritic impact.
You may be looking for something like the [Garrotxa Volcanic Zone].(https://en.wikipedia.org/wiki/Garrotxa_Volcanic_Zone_Natural_Park).
Santa Margarida volcano looks very much like I was envisioning.