Decorate wood retaining wall?

Amazon has some beautiful outdoor wall art. You should be able to get an idea there. The selection is really nice.

https://www.amazon.com/outdoor-wall-art/s?k=outdoor+wall+art

you could paint it a nice color like something in the blue family or aqua colors- you could add hand drawn flowers on it as well or paint each one a different color- you could plants that would droop down

I would freshen up the wood with a new coat of stain and then plant trailing plants that will drape down in front of it, that would be so beautiful. Potato vines and creepy jenny are two plants that trail.

You could hang mirrors on it too - a bit like this

https://www.hometalk.com/diy/outdoor/garden/our-mirror-mosaic-fence-we-had-a-blast-creating-this-amazing-what-can-come-from-a-192477

I dont think it’s ugly at all! What about putting some things in front of it? Like a bench or some big pots with flowers. Or a fountain like we made: https://www.frugalfamilytimes.com/search?q=Fountain&m=1

I love your shady private space and the mossy deer. Since the space is shady, I think the wall would look good stained a warm color. You didn't ask but the large pot might look good over in the corner with more red geraniums and the beautiful mossy deer could be up in the woods. You need just a couple of larger things up there rather than small stuff.

I would stain the wall and then add colorful planters with flowers :)

I agree that staining the walls and adding planters with vibrant plants (like geraniums) would help.

You could sand and stain it or even paint it with a bright color.

it all depends how much sun light you have out there ... but brick veneer would of look awesome