I'm looking for a simple, fast, easy notation software. The goal is that I can very quickly input a melody line or riff, maybe overlay chord symbols on it, when learning new songs or writing them, so that I don't forget them.

I use MUP. No gui ... and for me that is just fine. Not quite free.

ABC is similar. And if you still want to pursue non-gui stuff there is PMW (Phillip's music writer) ... free, but again might be too much.

I was looking at musescore this morning as well. If you want a gui, etc. this might be worth looking at. Looks clean, etc.