Thanks RvR, 

I understand how to add to the menu types like 'topbar', 'site', and 'page'.  I know that I need to add an item to an 'entity' menu... specifically the blog and comment entities.  It wasn't obvious to me how to do this from reading the docs.  So I assume I can do this in my start.php:

elgg_register_plugin_hook_handler('register', 'menu:entity', 'my_entity_menu_handler');

then

function my_entity_menu_handler($hook, $type, $items, $params) {
    $entity = $params['entity'];
    $handler = elgg_extract('handler', $params, false);
    if ($handler === 'blog') {
        // do something here to add a new menu item.
        elgg_register_menu_item(???)
    }
}

​

Almost...

Instead of elgg_register_menu_item() use the ElggMenuItem class:

$items[] = \ElggMenuItem::factory([
       'name' => 'register',
       'href' => elgg_get_registration_url(),
       'text' => elgg_echo('register'),
       'title' => elgg_echo('register'),
       'priority' => 200,
]);

return $items;

Ah.. ok.  Thank you.. that worked!

Thanks Jerome!

@seri_ng, here is the documentation of how you should do it in the future. http://learn.elgg.org/en/3.2/guides/cron.html

Will be releases within 1-2 days

Sweet! Thanks Jerome!