Another good option for a very different grammar that can change your thinking: American Sign Language (or another sign language if you're not American). It's t

Another good option for a very different grammar that can change your thinking: American Sign Language (or another sign language if you're not American). It's topic-oriented like Japanese, but it's also a spatial language, which is bonkers. For example, if you're setting up a story about two characters, you basically indicate a point or direction to represent the various parties, and then you periodically shift the perspective you're speaking from.