<?php
$parents = get_post_ancestors($post);
if(count($parents)>=2):
	//Page is a grandchild, show highest level parents as menu
	echo '<ul>';
	wp_list_pages('title_li=&child_of='.end($parents));
	echo '</ul>';
elseif(count($parents)==1):
	//Page is a child
	echo '<ul>';
	wp_list_pages('title_li=&child_of='.$parents[0]);
	echo '</ul>';
elseif(wp_list_pages("child_of=".$post->ID."&echo=0")): // page is top level and has children
	echo '<ul>';
	wp_list_pages('title_li=&child_of='.$post->ID);
	echo '</ul>';
endif;
?>