The first place you can begin is the kitchen. The place where most families spend the most time. New counters and/or cabinets are easy to improve and don't require a lot of money. Just by changing the doors, or just changing the handles or knobs will add a great look. If you have wood cabinets, or laminate cabinets you can apply a fresh coat of paint, using the same color or changing the color. To go one step further, why not give the ceiling and wall a new look. By painting you can even do some different textures, and create a design. Don't stop there, you can even get a different look by swapping out the old light fixtures, for some new ones. Light fixtures, come in many different styles, from modern to antique. If you have a wood floor in the kitchen, paint it with a nice design to match your kitchen motif. In addressing the window area, add some inexpensive curtains, that will make a big difference. You can also add a pre-pasted border around the wall, or chair rail area. Another inexpensive tip is to change the kitchen taps, there are so many neat one's to choose from.
The next room, that you can spruce up is the bathroom. This room is probably the easiest one out of all the rooms, because of the size. Just by switching the toilet seat, to one alittle fancier, or even humorous is a good place to start. Also don't forget the toilet paper holder, they have one's that are stand alone's or wall mounted. The light switch cover is another thing that can be changed. They are no longer the standard white or gold plated, you can now get them in almost any decor style. Lighting in the bathroom is easy to change also, or add lighting above the medicine cabinet or bathroom mirror. Sometimes on clearance you can find bathroom vanities, if not see if you are able to paint them. Shower curtains, are the focal point of the bathroom if you have them, and there are ton's of designs to choose from. Even applying some tub applique's can make a difference.
General things to change or improve, is just changing the curtains, and bedspreads, throw pillows, and throw rugs. These are just some of the tips that can change the entire look of any room.
If you have a inexpensive tip to share, let me know.