How do I protect the front side of a wood retaining wall?
If the boards (&amp; posts) are properly pressure-treated for ground contact, then there should be no need to worry about them rotting due to contact with ground/dirt on either side. For what it's worth I think it looks fine as is, but if you choose to add fence boards, they would either need to avoid ground/soil contact for be rated for ground/soil contact in their treatment.

The garden boxes appeal to me, but that's because I'd grow stuff in them - if that's not you, then they might be a waste of materials. Depending exactly how/where they are built, I guess they might also reenforce the wall.