This very simple boomerang is only two layers thick and uses less than 5 grams of filament. 
If thrown correctly, it will fly around a two meter wide circle and come back to your hands!

Printing and post processing:

Print it at 0.2 mm layer height.
After printing, wait for the bed to cool completely before removing it. This is important: because the design is so thin, if you take it out of the build plate while it's still warm, the wings may deform in an unwanted way.

After removing the boomerang from the bed, use the edge of a table to make a slight bend downwards along the lines on the wings, like in the picture below.

How to throw:

Lightly pinch one of the wings and throw it in an upright, almost vertical position. Don't throw it too hard, spin is more important than strength.

This model, as well as the pictures, are for a right-handed person. If you are left-handed, mirror the model using the slicer, so the line will be on the opposite side of the wings.

Have fun!