Try Nikon's 18-200 Zoom with Vibration Reduction. They're around $700 which is pretty good for a lens that can serve most all purposes and has image stabalization built in. For around $300, you can buy a nice 18-135. It's quite sharp but no IS and not so far from what you already have.
Now Sigma and Tamron both make fairly nice 18-200mm zooms at around half the price of the Nikon lens. They focus a little slower, don't have image stabalization, less optical quality, and don't have the build quality of the Nikkor, but if money is important and you use a tripod or good holding technique, the Sigma or Tamron offer the same zoom range at half the price.