The four ghosts are all symbolic of important eras in Scrooge's life.
The Ghost of Christmas past symbolizes a past where Scrooge was still joyful and hopeful about life. A plant of some sort might be a good object for this ghost, especially flowers or some holly, as these were objects the ghost was wearing.
The Ghost of Christmas Present is symbolic of how the other people in Scrooge's life are celebrating Christmas, and how Scrooge is missing out. Food or party favors might be a good symbolic object for this ghost, as the ghost shows up with an abundance of food and drink.
The Ghost of Christmas Yet to Come is symbolic of Scrooge's bleak future, with Tiny Tim dead and a very dark outlook if Scrooge continues on this path. A scythe (what the grim reaper carries) or a crutch (symbolizing Tiny Tim's crutch) might be a good object for this character.
The other ghost is that of Jacob Marley, who is there to warn Scrooge to live his life differently. A great object for Marley might be chains, as Marley is weighted down by chains that he created for himself by tying himself to the importance of money.