What are the kinds of things you are interested in? Do you want to experience some big city that you haven't been to before? Do you want to head out to the mountains? Would you want to see the Gulf Coast, or the desert Southwest?

These are all viable options with 20 days time - in fact, you could pick from quite a few different places and still fit them in. Figure 5 days from one coast to the other, so that's ten days, giving you another ten days to explore around.

What is "perfect" varies from person to person.