On top: peppers, carrots, beets, leeks, onions, garlic, eggplant, turnips, tomatoes, amaranth, and more.
Most other plants will grow from the side holes. Vines such as summer squash and compact melons grow nicely from the bottom holes, trailing onto the ground.
Balance large bushy or tall plants with compact plants to create a mosaic garden for best yield (three cabbages or broccoli cannot grow next to each other, but they can grow very nicely surrounded by lettuces or other compact veggies). Also pay attention to the rate at which various plants mature compared to others. For example, planting a cabbage which takes about 10 weeks to mature will allow for leafy green production (4 weeks) in adjacent openings until the broad-leafed cabbage overtakes the lettuces (light competition). By that time the lettuces will be near then end of their productive life. After the cabbage produces a yield you can start all over with the seeds you started for summer!