What is it that you perceive as insecure about the use of the $_GET array?
If it is being used to change the state of the server (e.g. inserting data into a database), then that is not a good idea. However, if it is being used to select what content to display and/or how to display it, then, as I see it, this is exactly what $_GET is for.
The advantages of using the $_GET array are simplicity, and that a static URL is automatically provided to the user (e.g. http://www.example.com/page.php?content=cat&theme=cute
), allowing them to link to / bookmark the page. The exception might be a page that you do not want someone to be able to go to directly (e.g. one requiring login). Using it for indicating what menu/submenu to display isn't much different, IMHO (.../page.php?cat=cat&subcat=siamese