I would suggest a plastic pot that fits inside only because if it should get too wet, you can remove it and allow it to drain (unless your concrete urn has drainage holes, in which case the point is moot). My sister has a pair of these on either side of her front window and she has a tall grass-type plant in the center, with a few pansies and varicolor ivy in each one. They look very nice.

Thanks Leytonlady! Yes, the urn does have a good sized drainage hole in the bottom. Having never done this before (planted in a concrete container), I was wondering if I would be able to keep the plants moist enough in this type of container.