Putting a crisp, white linen table cloth on a dark wood side table for the summer can also lighten a room and give it a great new look even if it's just a large square card table-size table cloth on a round table, it will work.
Add fresh flowers on top of the table cloth and wow! It makes a huge difference. Clean the fireplace and replace logs with silk greenery. This will make a huge difference in how the room feels. Put fresh paint on drab walls. Outside, remove brown leaves or plants that are struggling with fresh planted flowers. Get fresh seat pads or covers or slipcovers for your outdoor patio furniture.
Cover an old iron chair with a light French toile and you'll have an instant facelift. And don't forget to clean your windows! Have sparkling windows for those summer days.