So, my first attempt at filling these was a bust! I put a combination of poinsettias, red petunias and white begonias, for Christmas.
@Cinta, you were right, it was hard to keep them watered, even in the winter when it's not so hot.
I came up with another idea though - I planted Fireball bromeliads in them. They will stay small, and they'll turn more red as they get more sun. I think it looks pretty good, and I won't have to worry about keeping the soil watered. As long as I make sure that the broms have some water in the cups, they should be OK. Fingers crossed!