If you're looking at maybe changing the header (above the breadcrumb, navigation bar) you could use the Display Options -> General -> HTML Includes section. In the Default Header, you could do something like:
<?php
if ($user['USER_ID']) {
echo <<<USER_IS_LOGGED_IN
This will display to a user that is logged in.
USER_IS_LOGGED_IN;
} else {
echo <<<REGISTER_PRETTY_PLEASE
This will display to a user that is not logged in.
REGISTER_PRETTY_PLEASE;
} // end if
?>
