When I created the navigation menu at the top of my site using the menu feature in WordPress (Appearance → Menus), I didn’t want to have to maintain anything but the top-level menu items. When I released a new plugin and created a page for it, I didn’t want to have to go into my menu UI and add it there too. So instead I wrote this plugin.
Add Descendants As Submenu Items adds a checkbox to each menu item that is of a hierarchical post type (i.e. pages). Checking this box will automatically display all descendants as submenu items on the front end of your site. You can see it in action at the top of my site — I only configured the top level menu items and everything that shows up when you hover over them has been added by my plugin rather than manually by me.
For more details and to download the plugin, check out the plugin’s homepage.