Here are a couple of cute ideas for painting some relatively easy things on a children's or playroom wall. I like the lines drawn on the wall in this first picture. The sheep look pretty easy to paint. Another idea would be to paint simple shirts or pants on the lines like a clothes line.
The tree is an idea someone used for a "family tree." But I think it would also be very cute without the family tree part. Maybe just a couple of trees with the bed in between.