Beginner project ideas

Hey everyone, Im a student pressuring a mechanical engineering degree. I just completed my 2nd semester. I’m looking for a good beginner project to start on in my own time to learn some basics.

Things like springs, cams, motors, etc.

I need them to be relatively cheap

I already have a 3d printer and an arduino.

Any good ideas?

(I like cars and robotics the most if that matters)