If it's a big container you could try Amelanchier lamarckii; many Acers are perfectly happy in pots, but do better in ericaceous compost, and there is a pretty variegated Willow (Salix Hakuro wikishi) that is often sold in Garden Centres in containers.

I had a Rowan tree in a pot for 15yrs then I did put it in the garden.

I would say Italian Cypress and Buxus do well in pots and as are evergreen do not look boring for winter.