Extending the demo
Many demo discs have hidden extra content on them. Rather than remove everything that’s not part of the preview, developers add superficial limitations to prevent you from accessing certain game features. A little bit of reverse engineering can often let you explore beyond the restrictions.
Furthermore, a lot of of demo discs are built before the final game. They often therefore have extra debugging features present and references to elements that aren’t ultimately released.
Here is a collection of articles I’ve written about patching demo discs to allow exploration of prototype builds.