Enable window-numbering-mode and use M-1 through M-0 to navigate.

If you want to affect the numbers, use window-numbering-before-hook or
window-numbering-assign-func.
For instance, to always assign the calculator window the number 9, add the
following to your .emacs:

(setq window-numbering-assign-func
      (lambda () (when (equal (buffer-name) "*Calculator*") 9)))