<?php
add_filter( 'wp_get_nav_menu_items', 'cpt_archive_menu_filter', 10, 3 );
function cpt_archive_menu_filter( $items, $menu, $args ) {
    /* alter the URL for cpt-archive objects */

    $menu_order = count($items); /* Offset menu order */

    // We'll reorder all items and add new ones, so we want to have an independent counter
    $curr_order = 0;
    $new_items = array();
    foreach ( $items as &$item ) {
        $item->menu_order = $curr_order;
        $new_items[] = $item;
        $curr_order ++;

        if ( '##post##' == $item->title ) {
            $item->url = get_bloginfo('url') .'/growing-new-ideas/';
            $item->title = 'Growing New Ideas';

            foreach ( get_posts( 'post_type=projects&showposts=6&orderby=menu_order&order=asc' ) as $post ) {
                $post->menu_item_parent = $item->menu_item_parent;
                $post->post_type = 'nav_menu_item';
                $post->object = 'custom';
                $post->type = 'custom';
                $post->menu_order = $curr_order;
                $post->title = $post->post_title;
                $post->url = get_permalink( $post->ID );
                // Add a custom class - you can use this to indent the item, or add special styles
                $post->classes = array( 'project-link' );
                /* add children */
                $new_items[] = $post;
                $curr_order ++;
            }
        }
    }

    return $new_items;
}