Your plan is just fine. Mt Cook itself is incredibly scenic, and you will see the turquoise glacial lakes (which they don't have at Ruapehu).

Be sure to stop and buy some of the best smoked salmon in the world while you're up there. http://www.mtcooksalmon.com/visitus.htm

There really isn't a way to see everything in NZ in one trip. You have to make choices and hopefully go back (I've been 5 or 6 times, lost count!)