todoo.el by DanielLundin is very simple and based on OutlineMode. It is included in the emacs-goodies-el Debian package (DebianPackage:emacs-goodies-el).
I have the following function bound to f10 so I can press one button, edit my TODO file, press the same button and I’m done. I need my TODO file very close, or I’ll stop using it.
(defun todoo-or-close-todoo()
(interactive)
(if (eq major-mode 'todoo-mode)
(call-interactively 'todoo-save-and-exit)
(call-interactively 'todoo)))