Top digital camera settings Nikon D5000 Suggestions Looking?

I'm shooting with a Nikon D5000 and am really not happy with the quality of my photos. I have the 18-55mm and 55-200mm lenses. Tomorrow I'm going to a garden with lots of scenic opportunities and was wondering if you could give me some advice. The day is supposed to be sunny and I would like to come home with more than washed out colors. While shooting I can see the exposure is to bright or dark but can't compensate while in the scene modes or shooting straight auto. What would you recommend in lens choice and shooting mode?