If it's a nice sturdy and large arbor, I highly recommend yellow lady banks rose. Of course it only blooms the one time, in early spring (mine just finished blooming) but it sure puts on a show then. I've got bignonia 'Tangerine Beauty' growing with mine and they look really good together, although the bignonia blooms just a week or so later than the rose, but that's ok. Also I put on the other side last year a montana clematis, 'Rubens, maybe? pale pink with yellow stamens, that also filled in and looked very nice on top of the yellow roses, blooming at exactly the same time. Perfect! I like the smaller-flowered clematis with the small-flowered lady banks rose.

Of course my arbor is way too small, and I know that in future years something will have to be heavily pruned. In fact all three of them are probably going to have to be heavily pruned, as they all are very vigorous growers. But for now I'm very happy, and I think next year we'll be ok, too.

After that, maybe I'll have rooted cuttings for future plant trades, if I'm lucky.

And during the rest of the summer it's just lots of greenery. I tried letting a mandevilla vine grow through it but it was just a pain in the neck picking the vine out in the fall, as it's tropical and dies at the first frost. This year I'll just hang a painted birdhouse underneath the arbor for decoration. I also have pots of flowers all around the base of it.

You might try clematis armandii, which is an evergreen plant. It has pretty, dark green strappy leaves and it blooms with lovely white, fragrant flowers in the spring.

I put a passionflower vine on my arbor last year. Its crazy purple flowers and fruit looked great the whole summer and it stayed evergreen all winter. Totally recommended.