How would you decorate a wall to hide marks?

What type of marks?

How large are the marks?

I assume that painting is not an option for you. For a less expensive solution, go to a few thrift stores and purchase a variety of frames. Then paint the frames the same color to get a cohesive look. You can add your own artwork to the frames or find some posters / maps / whatever to create a wall of art. I would love to see what you come up with!

wallpaper, magic eraser,

I guess it depends on how high or low the marks are. How about wallpaper? If you are a renter I believe there is a type that can be easily removed if needed.

If they are knicks in the walls....go to the store and buy artist paint in little bottles...water based...cheap...the color of your walls or mix to match. If there is just dirty spots on your walls....magic eraser works super.