We will see Shelob take down Frodo and Sam picking up Sting and going into a rage and kicking her butt and then taking the ring and kicking butt over all the orcs that capture Frodo. And of course using the phial of Galadriel to "be a light in dark places"
(when Sam kicks butt is my favorite part of the whole series.)
We will see Saruman cast down before Gandalf, like in the book. Wormtongue will lose the Palantir and then Saruman will berate him and he will snap and kill Saruman and then one of the good guys will take him out as he flees.
We will see Aragorn and the Paths of the Dead and I believe it will be Arwen who brings him the sword.
We will see Aragorn confront Sauron through the Palantir and have a virtual showdown of wills through it. I have already heard rumors that PJ plans on physically showing Sauron in the third movie and this would be a perfect place to do so.
We will see Eowyn and Merry kill the Witch King of Angmar
We will see Gandalf and Pippin and the whole Denethor incident and the great battle of Pelennor fields.
Then we will see them all approach the black gate and face the mouth of Sauron and the armies of Sauron and the fight will begin. Things will look bleak for the heroes and then we will skip back to Sam and Frodo and Gollum at Mt. Doom. Frodo and gollum will struggle and then Gollum falls in and the ring is destroyed.
We will jump back to the battle and as soon as the ring is destroyed the tide will turn, evil loses and Sauron is destroyed in some impressive pyrotechnics.
Then there will probably be about 20 minutes or more of wrapup showing the main characters saying goodbye and the hobbits going back to the Shire. Then probably a bit about Frodo going across the sea and then later Sam and the film will end with appropriate haunting music