give defined time, create define 3D parts and 2D drawings from 3D, then check the accuracy, if the parts and drawings create according they learned knowledge already in usual, at the end give out a comprehensive score.
Or let them create the 3D model according invention anything what they want, and give some time to presenting it why and how to achieve this.
Make it an environmental problem that is a concern in your area or globally that can be used by anyone but be affordable. Such as plastic recycling, water clarification, sustainable agriculture, and many more. This will help them understand the design process, research and environmental impact at a simplistic level.