I have seen great shots straight from the D90. You should be able to set up a custom picture setting that allows you input contrast, sharpness and colour parameters that will give a more satisfactory image. If you set your camera up to shoot jpeg+raw you can then tweak the raw photograph that your not quite happy with later and convert this to jpeg to print.
Also raw allows you to set your white balance later if need be. For a start set your white balamce to your shooting situation. If outside in daylight, "auto" or "daylight" should be good. It is probably when indoors that setting it to the type of light such as "tungsten" or "incandessant" that you will see more natural colours in your images. A lot of DSLRs struggle with auto WB in these conditions, but again if you have the raw file you can select the WB on the raw file later, though setting it in camera first may save you time processing. Hope this is of some help