I can see the film ending rather well with the heroic death of Boromir, his funeral and the decision of Aragorn to go after Merry and Pippin. Frodo will have already explained his decision to Sam to go on to Mordor alone, and that should tie the whole thing up nicely and draw interest for the next film to answer the question of the Ring and the chase.

While that would be a dark ending, it's not as dark as the book ends, which seemed to my young mind to be the end of all hope. The fellowship is broken, Frodo and Sam are certain to be eaten up in the wilds (and thus the Ring will be lost), Merry and Pippin are lost to the orcs, and Aragorn, Legolas and Gimli are left with the worst of all possible decisions: Who do we abandon and who do we try to save?

-RobRoy