Here are some container gardening tips for you:

Try using the largest container you have. A saucer under the pot helps
stop drips and saves water. Peat moss will help retain moisture. So
will Soil Polymers. They can absorb up to 10 times their weight in
water. And don't forget to mulch.

You'll have to tie the vines up on stakes, or a trellis of some sort.
When the melons start getting big, you can slip them into the foot of a
pair of panyhose. Tie the stocking to the trellis. It will expand with
the melon and keeps pests away too.

For other plants to try up there, look for bush varieties. They have
bush versions of tomatoes, watermelons and cantelopes. Peppers are
bushy anyway and would be happy in a 10 gallon container - use lots of
mulch.

Good luck, Laura