The obvious answers are kitchens and baths, but a more subtle, less expensive and highly desirable change is taking out carpeting in common living areas (OK to leave in the bedrooms) and replacing it with a wood floor, or wood laminate. Dark floors are very popular right now. Finish it off with a nice 4-5" tall white base molding and you get a polished look that feels like new construction.
Don't go too cheap on the laminate - it will sound cheap when you walk on it and easily gouge. Depending on the price-point of your home, I'd opt for a real wood product fused to a compressed base. Pre-stained will be cheapest/easiest. Some of the real wood toppers can even be sanded and re-stained down the road. If you have big dogs with sharp claws - maybe not a great idea.
Also, large 2'x2' square travertine or ceramic tiles can offer a similar new look and feeling to a home. Just be warned tile can be cold to the feet and harder on little kids and their falls.
Just be sure to pick a nice neutral color that buyers will like, and blend it nicely with bedroom and bathroom flooring.
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