Decorate old acoustic guitar for wall hanging

Depending on your room, you could use stencils for designs or if you want a bold look, you could go with unicorn spit. Have fun!

You can take the face/top off of it and put shelves inside the body of the guitar which can make it functional as well. If you leave it intact, you can put LED lights inside (work from the back) -- color changing ones would look neat. The light will shine out through the center hole, but you could add some decorative holes as well in top.

How about making it into a planter? Glue a pot that doesn't have a drain hole inside, perpendicular to the hole in the center, fill with fake plants or if using a live plant, put about an inch of gravel in, potting soil, plant. Water carefully to avoid spill over and/or drowning plant.