If you're searching for a project for students in 3-D Geometry, why not have them sort prisms and pyramids? It took me a bit to get this program to run the way I wanted it, but some extra effort paid off. I think students would be able to create something very similar.
Experience has taught me that students really struggle with finding the perimeter or area of a rectangle. Why not give them an opportunity to show what they know by coding a program that calculates perimeter and area when length and width are entered? Here is an example of what two students coded. I love how she and her partner took the math concept and then added their own twist to it. (Please excuse the spelling errors!)
A simple project to start off a class would be this one: code a program that adds two numbers together. Students learn how to use a variable and get a kick out of creating their own calculator.
I told my students to create a program that draws acute, obtuse, and right angles as a way to review what they had learned in this unit. They also showed a lot of problem-solving skills along the way.
I designed this program as a starting point so students could show what they understood about the probability terms certain, impossible, equally likely, likely, and unlikely.
https://scratch.mit.edu/projects/159904495/
I was blown away how students took this assignment and changed it something much more interesting than I had dreamed up! Take a look.
https://scratch.mit.edu/projects/161021649/
The trick I discovered while coding this program, is that when you are creating your reflected sprite, be sure to crop out the background. Otherwise, your reflection won't be symmetrical.
https://scratch.mit.edu/projects/160146026/