just a guess
I think a really great stopping point for TTT would be as Frodo, Sam, and Gollum watch the army of Mordor leaving Minas Morgul.
In the book Frodo almost falls into despair as he watches the host of Mordor leaving to assault Minas Tirith. This is also the last chapter before Gollum leads Sam and Frodo into Shelob's lair.
I think the visual of the might of Sauron's army marching out of the black gates of Mordor would be an impressive sight to see as the screen fades to black and the credits roll.