functions.php

<?php

function random_post() {

  global $wpdb;

  $arr = $wpdb->get_results("SELECT ID FROM wp_posts WHERE post_type='post' and post_status='publish'
    ","ARRAY_A");
 
  $curr_id = get_the_ID();
  $numbr = array_rand($arr);

  if($arr[$numbr][ID] == $curr_id){

  $numbr = array_rand($arr);
  
  }

 $new_id = $arr[$numbr][ID];
 $postlink = get_permalink($new_id);

 return "<a href='" . $postlink . "' title='" . get_the_title($new_id) . "'>Random Post</a>";

}

functions2.php

<?php

add_filter('wp_nav_menu_items','add_custom_in_menu', 10, 2);
function add_custom_in_menu( $items, $args ) {
    // Don't forget to chnage "primary-menu" to the ID of your navigation menu
    if( $args->theme_location == 'primary-menu')  { 
		
        $items .=  '<li>' . random_post() . '</li>';
 
	}
    return $items;
}

sidebar.php

<!-- The code below will generate a list of 5 random posts -->

<li><h2>Random Post</h2>
  <ul>
    <?php $posts = get_posts('orderby=rand&numberposts=5'); foreach($posts as $post) { ?>
      <li><a href="<?php the_permalink(); ?>" title="<?php the_title(); ?>"><?php the_title(); ?></a></li>
    <?php } ?>
  </ul>
</li>