I don't do many because usually it turns into a parent project instead of a student project... but when I do, I always include activities that I know for sure the student did, like a writing portion where they have to explain their understanding.
Last week, my students had a home project to show their knowledge of simple machines by designing a leprechaun trap! (click the link for the freebie in my store!) They were SO excited about it! They only had to draw the designs and explain how they would work, but many of them built the traps too: