Tree Menu represents menu items in clear tree-like hierarchy, which is very appropriate for indexing menu. By default all submenu items are collapsed until you select the parent menu item.
To setup Tree Menu you just need to configure module parameter Menu Class Suffix appropriately and the menu system will take care of the rest.
Tree Menu with Rich Text
Here you can see live demonstration of Tree Menu with rich text.
Tree Menu can present menu items with descriptive text, pretty much like Main Menu.