write a random function
$file = "urls.txt";
$fp = file($file);
srand((double)microtime()*1000000);
$urls = $fp[array_rand($fp)];
echo $urls;
could do that and call it from
<a href="<?include "random.php"?>">
and if they must be .html pages you could add to .htaccess
RemoveHandler .html .htm
AddType application/x-httpd-php .php .htm .html .inc
dont know if thats what you are looking for or not