At school level, a project in mathematics subject could mean do an activity that demonstrates mathematical laws, formulas, derivations. We make a physical model that explains some principle.
The activity of taking an idea, planning what to do, how to realize a physical model in terms of a model in paper or other material. This activity of some persons converting an idea into a physically seen model will be a project.
It could perhaps be simply demonstrating complex shapes pasted on posters, use of statistics or probabilities. It could be demonstration of mathematics with playing cards. It could be study of properties and uses of prime numbers. Well arithmetic progression and its uses can be shown.
Savings in bank, compound interest gained over a long duration - can be presented in an interesting way. Savings and loans - how it works out.
Simplest one: you show how Pythagoras theorem is true. You can draw a square on each side. Then you can lay (arrange) the squares of other sides into the square of hypotenuse. They should match. Of course, you may have to cut one square into pieces.
You can make a sun dial - with the length or position of shadow denoting the time.
The main idea is to convert some idea into some thing physical in a coordinated manner and present it coherently after completing it.