When creating content, I'd like to have something akin to a drop down menu. You could hover over an item and a list of checkboxes would appear. Essentially it'd be a Select List with sub-items. Is there any easy way to do this?
You can try Hierarchical Select module. Project page says
This module defines the "hierarchical_select" form element, which is a greatly enhanced way for letting the user select items in a hierarchy.
It will give you a way to select hierarchical items using select boxes.