What do you think my first suggestion would be? Paint, of course! Paint something a fresh, vibrant color! Try taking your regular room palette, and just punch it up a bit! Or, add co-ordinating brights. If not your walls, how about a door, a piece of furniture, or a few accent pieces. Try french or periwinkle blue, sunny yellow, lavendar, or fire engine red. Save the more intense colors for smaller areas, of course!
Fabric is your next choice! To avoid the high cost of designer fabric, how about dyeing cheap muslin or cotton the color of your choice. Another lower cost option? Buy bed sheets, especially at the Labor Day white sales! Use fabric panels not just on the windows, but hung on either side of a doorway, behind a bed or other focal point, tablecovers, slipcovers, even used on a wall like wallpaper. For the latter idea, either hang a rod the length of the wall, and hang fabric floor to ceiling, or paste to the wall with liquid fabric starch. This option is great if you rent, because it can be easily removed when you leave, with water.
Save clear bottles and fill with colored water, set on a windowsill and let the sun sparkle through!
Recover picture frames or mats with a colorful fun fabric for the summer... easily removed when winter returns.
Frame and hang childrens' drawings in bright, primary colors! This is especially great for an all white kitchen, and adds a personal touch that makes home special.
Pot up flowering annuals as houseplants... they are cheap and available this time of year, but can be grown year round inside.
Sew bright, fun covers for your throw pillows. Make them with envelope openings in back, and they can easily be removed and used again next summer!
Bring fresh flowers in from your garden! They always look better (like most things) if they contrast with the background, which is why the most beautiful vase of flowers often looks lost on that windowsill. Try putting them in front of a solid suface, and watch the flowers glow!