Render the entire menu tree in a Drupal theme. From http://drupal.stackexchange.com/questions/4656/show-all-nested-menu-links-in-drupal-7

Render theme menu.php

<?php

function THEMENAME_preprocess_page(&$variables) {
  // Get the entire main menu tree
  $main_menu_tree = menu_tree_all_data('main-menu', NULL, 3); // Provide max-depth.

  // Add the rendered output to the $main_menu_expanded variable
  $variables['main_menu_expanded'] = menu_tree_output($main_menu_tree);
}