This nice object casts 3 different shadows: a circle, a square and an equilateral triangle (the 3 Bauhaus essential shapes). It's important to note that this is theoretically truth only when light rays arrive in parallel (which requires a punctual source of light infinitely distant). In practice Sun rays will do the job perfectly, whereas artificial lights won't.

There are many more objects, often more complex than this one, that cast interesting shadows. Check, for instance, this one: https://www.thingiverse.com/thing:202774

Also very famous is the EGB objects, featured in the book cover Gödel, Escher Bach, and found among the examples in OpenSCAD.

(I printed a version with some rice on it (I used a straw to slide the rice while printing) and made a nice maraca).