Hang wall art. You can find a variety of pictures, appliques and stickers representing sports. These apply directly to walls. You can accent them with ceiling borders featuring favorite sports. Tack up posters of sports stars on the back of the door.
Put a sports shade on the lamp. Add a matching bedspread. You can repaint a desk chair with referee's stripes, and you can attach a field goal to the waste basket for those long shots with paper balls. You can even buy soccer-ball knobs and replace all the old knobs on your child's desk drawers.
Replace the old light fixture with a hanging football or soccer ball. You can add stickers depicting sports scenes on the ceiling around the light fixture. Mount a miniature basketball net near the ceiling for shooting practice.
Pin up team banners on the walls. You can order these from any team's website. You can include college sports, pro sports and amateur sports. Make an arrangement of these above the bed on the wall.
Mount sports equipment on walls. Hockey sticks, shoulder pads and baseball bats can make attractive wall arrangements. Vintage sports equipment from garage sales can add a touch of sports history.