A small hollow classical guitar, just a little smaller than 16cm. Only four parts to print and few drops of glue. Could be a simple assembly projects for young kids.
Instructions
Can print on a 10x10cm printer bed.
Use 0.2mm layer height or better.
Print "Body" and "Top" with one color, "Bridge" and "Neck" with another.
Glue "Top" to "Body" with few drops of Super Glue (CA).