6" x 1" cedar fence boards have worked well for us with 2" x 2" posts in each corner and at the mid point. Provides a 4" high or 9" high bed depending on whether you make the bed one board high or two. Ours are into their fourth year and are still going strong. A cheap and good looking solution for this area.

If you live in a mild climate where the ground doesn't freeze deep and you would like a permanent solution, then poured in place concrete might be a good choice.

No toxicity worries with concrete. It's permanent and you don't have to be careful not to damage it when working the soil with shovel or tiller. It adapts well to slopes to make terraced beds. And cost can be less than some wood alternatives.