Since most of your working hours are spent in your home office, try to design it so that your productivity is maximized:
Use bright colored paint for walls - it will make the room look bigger.
Taping off sections of the wall in your office and painting each section in a different color.
use collages of pictures of landscapes, flowers, animals or anything you like, arranged artistically .
Have a mural painted on the wall if possible
Picture frames
Live plants
Wall paper borders can be added to top/middle of walls
Memo/pin boards - can be made yourself using sheets of Styrofoam panels rectangles or squares