2ft x 2ft x 2ft planter box - how to join the 2x4s?

[tenders]

You could also use a "cleat" of 1x1 on the inside of the joint. Nestle the cleat in the inside corner of the joint, and screw through the edge of the cleat into your 2 x 4s. That's a pretty strong joint and you probably won't care that the cleat protrudes inside the box; also, there will be no external fasteners showing.

If you want to overkill, apply epoxy to the cleat before you screw it in.

I agree - use PT lumber and stainless fasteners.