The easist is to sell it or donate it.
But you could also:
Music player.
Game player.
Main alarm clock.
Those are my thoughts for my old D1.
But also...
Clock.
White noise player to help you fall asleep (some of the free ones don't shut off or don't go back to the home screen or whatever--well the one I tried didn't).
Kids toy. Add a bunch of kids games on it for them to play.
I just googled and not much different then what I wrote except, 911 still works on old phones. So you could use it as any of the ideas above, just keep it charged up for an emergency.
After having the Eris for 2.5 years.. its finally time for an upgrade!