public function activity($username, $request) {
        $options = array(
            'user_did', 'happened_to_user', 'contacts_did', 'happened_to_contacts', 'everyone_did'
        );
        if (!in_array($request, $options)) {
            throw new Garp_Service_Vimeo_Exception(
                'Invalid request. Available options are ' . implode(', ', $options)
            );
        }
        return $this->request('activity/' . $username . '/' . $request);
    }