Basically stay off the interstate.
Depending if you're going to Sandpoint or Bonners Ferry (make sure you get ahold of Roadie),
here's one good way.
Here's another.
Of course, these are somewhat direct, as direct as you can get avoiding the superslab. There are many other detours if you've got time. If you're going through Coeur D'Alene, there's a TON of cool little roads. Just let me know where you're going roughly. Freedom and Roadie can help, too.