se-random-post.php

<?php

add_action('wp', function() {
    if( get_query_var('se_random_post') ) {
        $post = get_posts(array('posts_per_page' => 1, 'orderby' => 'rand'));
        $post = array_shift($post);
        if( $post && $post->ID ) {
            wp_safe_redirect( get_permalink( $post->ID ) );
            exit;
        }
    }
});

add_action('init', function() {
    // rewriting site.ext/index.php?se_random_post=1 to site.ext/random/
    add_rewrite_rule(
        'random/?$',
        'index.php?se_random_post=1',
        'top'
    );
});

add_filter('query_vars', function($vars) {
    // saving the query variable for the rewrite rule
    $vars[] = "se_random_post";
    return $vars;
});