Ghosts definitely make "A Christmas Carol" a fantasy. Ebenezer's good friend Jacob who is deceased comes to visit him. Ebenezer cannot believe what he is experiencing but he is sad to see Jacob shackled and looking tattered and worn.
Another element that makes this story a fantasy is the fact that at the end, Ebenezer doesn't know whether what he has experienced is real, but regardless it has changed his outlook on life.
Lastly, there is the element of fortune telling. The Ghost of Christmas Future shows Ebenezer what is to come if he doesn't change his ways.
Sure the presence of the three ghosts of Christmas past, present and future could be considered one element of fantasy. Secondly, the idea of being able to see into the past or future scenes in Scrooge's life and those around him. Thirdly, I'd say the fact the Scrooge is flying through space and time with these ghosts to reach the scenes of past, present and future, that he is able to be taken there even though he is not a ghost, represents another type of fantasy.
In Charles Dickens play and story A Christmas Carol Scrooge's whole experience is a fantasy as stated. The first evidence is the entrance of the ghost. The ghost carries the chains and clanks around. His presence is that of his former worker Jacob Marley.
The second evidence are the three spirits. He is transported through time (also a fantasy) into the different times in his life. In the past he sees what he ahd wanted but let go. In the present he sees the sufferings and joys of those who work for him. In the future he is able to see what he will become.
Like in the story, The Wizard of Oz, he wakes up to the morning after he had gone to sleep. The reader is aware that it could have been a real experience or just a dream which is a fantasy like state of being.