Do you know your zone? Are you going to leave them outside for the Winter if your climate does not freeze and thaw?
If you are going to leave them out for just summer. Moss Rose would look good in them. They like a dry soil. Sedum would look good. They come in pretty red, yellow colors.
If you have winter climate and you want to leave them outside for the winter I would do what I did above and use the semps (Hens and chicks), mixed with hardy sedum.
