Wouldn't want to just lay them in the ground and fill them, you'd want to dig a hole and submerge probably half of them, then fill the bottoms with a mix of the ground soil you dug out, and soil amendment meant for raised beds. This will help to deter subterranean pests from getting in.
You may also want to (carefully and with the right bits) drill 1/2" holes spaced around the outside to let more water drain and let microbes and worms in and out. This helps to normalize the inside with the outside soils. Then fill to about 1" from the top.
As for what to plant, I would imagine leafy greens that like cooler conditions would like this sort of setup (the soil inside will stay cool versus a wooden raised bed structure). Spinach, Kale, Cabbages, Mustard Greens, Brussel Sprouts...etc.