There isn't an out-of-the box solution to this, at least not to do it automatically.

There are web parts available from various sites  that will provide tree listings of sites that have been created and one of these might suit your needs. Google should be able to help you find most of them.

If you don't want to add a web part, the only other suggestion I can make is to manually add listings but that doesn't meet your criteria of "automatic".