A raised and portable salad table would be great for a balcony, rooftop or yard, a good way to grow shallow-rooted leafy vegetables like lettuces, spinach, and other greens.
The simplest designs use either wire or window screen material fixed to a wooden frame. Wooden cross-pieces can be used to add strength, and to divide the frame into compartments. A soil-less potting mix should be used since it's much lighter, and allows the plants roots to breathe better. A mix composed of course material, like perlite, some worm castings and peat moss would also work well.
For wood, 1" x 4" pine is inexpensive and works just fine, however you could also use wood from a pallet. Just be sure not to use treated wood, since this is for consumption!
See many more ideas for Salad Trays and Tables at Grow-it-Organically.
(Images by Steve Masley)