When you take a look at all the tools provided by GeoToolkit, you'll be quite impressed. For example, within the US map shown in yesterday's blog entry, you can drill down into individual states by selecting them via the mouse, as shown below:
With that, the basis of a more complex application is laid, since all the map-related functionality is handed to you out of the box.
The sample referred to yesterday has been updated, if you check it out and run it (assuming you've taken the additional steps mentioned yesterday), you'll see the above.