Because the US is so big, it's hard to plan a trip without some sort of defining factor. That could be a theme (historical sites, natural wonders, big city, etc.) or a region (New England, the Mississippi valley, Texas, etc.).

Having lived in the eastern half of the US my whole life, I made the mistake of forgetting how big the western states are when I visited the southwest. I spent a big portion of my vacation driving from site to site.