activate-wp-plugins

$wordpress_path = "/path/to/my/wordpress/install";  

// may not be needed if inside WP and hooking into actions
// activate_plugin() is in plugin.php
require_once( $wordpress_path . "/wp-load.php" ); 
require_once(  $wordpress_path . "/wp-admin/includes/plugin.php");

$plugins = array("cforms",  "w3-total-cache",  "wordpress-seo");
foreach ($plugins as $plugin){
  $plugin_path = $wordpress_path."wp-content/plugins/{$plugin}.php";
  activate_plugin($plugin_path);
}