Pallet gardening, perfect for gardening within a small space. Always take advantage of empty vertical space--it will leave your balcony or patio looking clean, not cluttered. I frikn love stuff made of pallets!
Good idea if you have rock solid ground that is hard to dig up. If you don't cover the holes you can use them for herbs & annuals. also lined with weed barrier cloth & used rebar in several places for stability. It makes maintenence super easy.