Buy a large free standing plant or two if you prefer (buy it in a garden centre or nursery more variety and cheaper), maybe paint it or paint two opposite walls to match the current colour to jazz up the room a bit. Buy a nice rug for the floor. Buy a small cactus for your desk (good for the elecro magnetic radiation from the comp screen). Think colour, add colour to the office maybe fresh flowers or artificial ones. Stick up a shelf with a plant and a few books, magazines, etc. Or buy a bookcase and put that there. Buy a nice painting for the wall, or buy a nice framed poster for the wall (an inspirational one like, courage, chellenge, etc). Add any qualifications you have, frame them and put them on the wall. Stick up a few framed photos of loved ones etc. Stick a picture of somewhere you always dreamed of going on holidays (good for motivation lol)
Just some ideas for you.