To me the most intriguing ideas that have been explored by Wittgenstein are related to his notion of a "private language" (and the question of whether it is simply possible or meaningful). You can ask for instance what is the difference between your body pain, which seems to be a kind of "internal object", that only you can possess and access, and the blue of the sky, when you look at it, which doesn't feel like an internal sensation (rather like an external fact), even though it can be construed as one, if needed.