Explore structure of a package, view its constituent classes, subpackages and dependencies using UML Class diagram. Right-click a package in the Project view, and choose Show Class Diagram on the context menu, or just click Ctrl+Alt+Shift+U.
You can also use the Class diagram to create a visual model, populate it with node elements and members, and draw links. IntelliJ IDEA will generate the source code, and keep it always in sync with the model.