Who Framed Roger Rabbit
There must have been earlier films that combined live action with animation. Did Mary Poppins do it?
Mary Poppins had them jumping into a picture scene and dancing with Penguins and horses. There was also Song of the South.
Terry Gilliam films all have this sort of otherworldly fantasy feels that are unique to him.
The Company of Wolves is also quite an interesting movie. Following into the fairytail dreams of a girl coming into puberty.
And if you want really surreal then The City of Lost Children.
Identity is also another interesting style. I won't spoil it by saying why though.