Assigning variables on layout/default.ctp

I'm looking to be able to set some variables contained within the layout/default.ctp file, which I can then assign using an included element.

The idea is that the site has a two-level navigation system. The primary navigation, a horizontal bar along the top of the site, will appear on each page. Each of these links will send the user to a specific area of the site, which will have it's own