Nasturtiums for any of your watermelons, squash or cucumbers. Potatoes? Haven't heard of any except maybe marigolds or painted daisies. I bet someone on this forum knows one, though!
I second the recommendation for beans & marigolds with potatoes. They also *look* pretty in combination (marigolds around the edges, then potatoes, then bush beans, then runner beans). I've been thinking about making a child's teepee garden with this combination so our son would have a little hideaway.