Cup & Saucer Vine is a beautiful alternative, with lovely cup shaped purple blooms... mina lobata, commonly known as Spanish Flag, is another lovely annual vine. Morning Glories come in so many colors that you could really make a nice statement with an assortment of colors. Hops can be a lovely vine to grow... cardinal vine, cypress vine, hyacinth bean, nasturtium, scarlet runner bean, sweet peas are a few suggestions... thunbergia, also known as black eyed susan vine is a swift grower...

Or.. if you wanted to go with a perennial, Trumpet Vine, a rambling or climbing rose, Honeysuckle, climbing Hydrangea, an Ivy...

Your choices are quite vast, really... and it's all dependent upon the amount of light you have to work with. Your zone would probably allow you to grow perennial vines without too much trouble with protection.

As for medium, I'm a big believer in using a more durable, free draining medium in containers... you'd have excellent success with vines or any plant if you used something like Tapla's 511 Mix, described in the link below.

If you make the effort required to begin with a good durable, free draining medium, and you keep the vines fed well all season, they'll develop healthy root systems, which will in turn make for healthy canopies and blooms on them, covering that fence in no time!

When it comes to annual vines, I usually plant groups of 2-3 seeds, just in case they don't all germinate... and in a half barrel, I'd probably plant 3 or 4 groups of 2-3 seeds toward the back of the barrel next to the fence, leaving a small area toward the front to stuff with other trailing or contrasting annuals... such as something in a complimentary color, or something like Dusty Miller or Euphorbia, or perhaps you could stuff a Daylily or other perennial in... like Creeping Jenny...

The sky's the limit, really... what colors do you see when you close your eyes and imagine what it would look like? Browse your local garden center for plant ideas... or use Google to see images of annual vines mixed with other plant types.

I hope this helps you somewhat... my personal favorite annual vine is Cup & Saucer.

Happy Gardening!

Here is a link that might be useful: Container Soils - Water Movement & Retention 13

This may be late but look into moonflowers. The foliage looks like morning glories but the flowers are big and white and only bloom at night which is kind of neat. The seeds are a little high maintenance to get going. Once you get a few going it's really easy to collect seeds for next year.