Bath tub refinishing, hands down.
Every woman who walks into a home (any home), walks straight to the bathroom (at least in her mind!) and checks out the tub. If she's disappointed there, she'll look on the rest of the viewing less favorably. Prices range from $300-500 per tub and the money couldn't be better spent.
This picture is of a more traditional tub but it works wonders in fiberglass surround style tubs and even when tile is involved. Check around for pricing - some refinishers will give you discounts on doing multiple tubs in one house as well.