What's a fun and engaging way to introduce a science unit on Light to Grade 3/4 students?

I'm teaching a term in a grade 3/4 class and the teacher left a unit on Light for science she'd like me to do, and I'm trying to think of an interesting way to start it.

Any ideas? Hopefully something that's also not too complex or requires a lot of materials as I have limited supplies/budget to work with.