I've never been good at keeping containers watered. They did a segment about this on Volunteer Gardener last year, and I remembered because I was tempted to try it.

This lady was really big on juniper. She planted some as specimens, and in other containers she had 2 or 3 different kinds (looked like they would outgrow the container pretty quick, but it looked nice). She said she rarely, if ever, had to water.

I also remember reading a story in one of my books about tough garden plants. The author said he remembered as a child visiting his friend's house, and the friend's mom had a big concrete planter full of sedum. Years later, it was still there. He said he had planted some in an old boot and left it untouched on his front porch for several years. :)

What about some kind of ornamental grass? They'd have to water that some, but maybe not really often.

Sedum or marigolds.

or...one of my favorites, Nasturtiums. =)

Hens and chicks! I just brought back a concrete planter from my aunts house that had never been watered. It found a nice home next to a glider swing.