WordPress load template part based on request (pseudo code)

load.php

global $wp;
$current_url = add_query_arg( array(), $wp->request );

// echo '<pre>' . print_r($wp->request, true) . '</pre>';
// Add logic here:

$load = locate_template( 'archive-activity.php', true );
if ( $load ) {
	// just exit if template was found and loaded
	exit();
}