Something I always find pretty mind-blowing is that a physical thin lens can also perform a Fourier transform.
This (generally) only applies to coherent light, so it's not something we're used to everyday, but it forms the basis of a lot of laser-based optical technologies. Combined with some other effects you can do "all-optical" signal processing [1,2], or generate create dynamic, "programmable" holograms [3]
I'm not an expert, but it's interesting.