Printed a Lithophane for my friends... They were amused. About 50cents in material, 6 hours to print for a 2" x 3" piece.
Essentially a b/w image where the software thickens the darks and thins the lights.
[googled image; not actually my friend]
More extravagant ones:
https://www.withgifts.com/Personaliz...BoCkI0QAvD_BwE
https://www.youtube.com/watch?v=YjkP1BcwLNI