Menu across just center column?
 

A typical menu goes across the whole page, as seen below.

Attachment 2561

What if I wanted the menu to stop at the sidebar? I know I can put widgets at the top of the center column, but how do I put a horizontal menu with drop-downs there?

You can't use the Atahualpa menu there, but you could add a new widget area to the top of the center area and add a menu plugin to it.