I am planning of trying these. I dont have much experience with them though.

-Vinca minor. Blooms may-june in Finland. Likes shade. Evergreen. I have heard it can invade a forest so should perform well 🙂

-Ajuga reptans. It should be aggressive and vigorous. Blooms may-june in Finland

-Lysimachia nummularia. Blooms july-september in Finland.

I have seen some forest floors with allium ursinum and it looks really nice. I would like to try that one also as suggested here.