Hey PV -- I just got a buddleiia for my columbines to shelter under (shhh, in the same container). I'm going to prune it up (if it lives) so it doesn't bother them too much...let's hope the battle for root space isn't too intense...

You need something tall because the columbines can get to two or three feet -- and something that has leaves above one foot so the columbines don't smother it -- and something that won't clash with their delicate foliage.

Annual, eh? I don't know too much about annuals -- what about snapdragons? They come in lots of colors ...and they'll be getting tall as the Columbine is fading... and fading when the Columbine (might) bloom again in the Fall...

What color are the columbines, or are they mixed? Pansies came to mind and so did impatiens, both hardy, woodsy kinds of annuals. Depending on the color, bachelor's buttons might work. I've never had much luck growing BBs, (bugs get 'em) but maybe it's just me.

An ivy or vinca vine would look nice to contrast both shape and habit. How big is the container? An Iris or maltese cross or yucca would give a neat tall contrast but iris are past their bloom here.