Create a new page with some extra information, so that your visitors don't ever plunge on to the default boring 404 - page not found error page.

Once this page is created:

► Remember its node ID,

► Go to Administer > Site configuration > Error reporting

► Set Default 404 (not found) page to the node ID you just created

► Save your settings

You can also use the Search 404 module as an alternative.