How can I decorate the wall behind a bed with no headboard?

Hang a pretty blanket or tapestry. If you use command hooks and a tension rod or a dowel you won't even need to drill holes

You could do a neat stencil pattern, check out cutting edge stencils they have a wonderful selection.

I agree that using Cutting Edge Stencils would be nice. I recently used their stencils to create a feature wall in my home's master bathroom: http://momhomeguide.com/currently-october-stenciled-wall-bathroom/

IMHO all beds should have back boards of some kind so that if you are sick you have a place to lean against and of course for those of us who can not sleep without reading first, a pillow to rest against. It also helps keep the wall clean. A simple board would work, an old door, a DIY head board. This is one I made for my grandson's narrow attic bed. The necessary components were all found including the shaped board. I had the batting and the fabric I got for free. Just stapled it around the shaped board.

You can stencil a headboard to the wall!