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.
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.