In Spacemacs, how do I insert a newline, without entering insert mode?

In Spacemacs, it is also possible to insert a new line while remaining in normal state with the wildly useful command spacemacs/evil-insert-line-below.

It is bound to SPC i j in Vim mode and M-m i j in Emacs mode.

Preceding that command with a numerical argument will insert more than one new line, e.g. 4 SPC i j will insert four new lines.

You can use:

(define-key evil-normal-state-map (kbd "RET") 'spacemacs/evil-insert-line-below)

to insert a line below and stay at the same position, or

(define-key evil-normal-state-map (kbd "RET")
  (lambda ()
    (interactive)
    (call-interactively 'spacemacs/evil-insert-line-below)
    (evil-next-line)))

to insert a line below and go to the new line.

I usually use ]SPC to append a new line below. It is equivalent to SPCij.

Also you can add one line above by [SPC.