I really like doing deep optimization work, so probably the GPU stuff was more fun from an engineering geek perspective. But the physics was also super fun, experimenting with how to model each object, and so on. The physics is a little more playful, since changing a tiny bit of simple math often does fun/weird things.

RE strings/air, I have thought about it, but only a little! Down the road I really want to explore more physics objects. It's very fun to think about how I'd integrate strings into the world, especially w.r.t. how they'd interact and connect with the masses. It seems like there could possibly be some very cool ways to do it.