That's colder than where I've lived before, but I think you could get started with Sempervivums (hens'n'chicks) and there's a few Sedums that I think are hardy that far north. Possibly some dwarf evergreens, but I don't get into those at all. A search here of specific characteristics for zone 5a, and clicking the "suitable for mini gardening" box yields 10 results.
As you find suitable plants that don't have the zone info already filled in here, it would be cool to help other gardeners out by adding that here. I've done that for the few that I've left outside that are still alive after a winter, but I can only say that they're hard to Z8b.
As others see this, hopefully you'll get some other suggestions. Best luck!!
