Here are some tetrahedral Christmas ornaments.
One has images of Koch snowflakes impressed into the surface and another has images of Christmas trees. The recesses can be filled with separately printed snowflakes and trees of various colors.
Stl files are:
Ornament_Snowflake.stl and Snowflake.stl to fill the recesses
Ornament_Tree.stl and Tree.stl.to fill the recesses
OpenSCAD files are included. ArchitectsDaughter.scad is from More Fontz by polymaker - thing:13677.
All the files can be printed directly on the build platform. I used ABS and a Makerbot Replicator with 3 shells and 5% infill. The ornaments printed in 2 hours.
The inserts are designed to fit snugly into the recesses. A small bit of adhesive can be used to permanently fix the insert.