Here is a cool nested drop-down navigation menu that uses only CSS for animation. It was designed by Gabrielle Wee. When you click on the menu button, the rest of the navigation menu drops down and there are some of those which can be clicked on to show the 2nd level menu items.
If you are having trouble with the pen, try the archived copy on GitHub
Drop Down Menu Snippet
See the Pen CSS-Only Nested Dropdown Navigation by Gabrielle Wee (@gabriellewee) on CodePen.0