People are talking about "giving away stuff" and "spoiling", but if you're making a demo for an adventure game, I don't think it should be a cut of what your actual game will be- that seems boring, and in that case, I'm not sure a demo is useful. I mean, sure, if you are going to have a year or something between releasing the demo and the full game, it might make sense to allow the player to retread stuff they saw in the demo.
But otherwise, while it is a little bit more effort, I'd suggest using locations and characters and music- all reused assets- and writing style of your final product, but have what actually happens be slightly different. Might even be cool to allow it to function as a prequel or sidestory (that doesn't take away from the main game if it is not played). I remember LucasArts adventure game demos used to do this, and it was always cool playing them, as they were their own little unique mini-adventures.
But otherwise, while it is a little bit more effort, I'd suggest using locations and characters and music- all reused assets- and writing style of your final product, but have what actually happens be slightly different. Might even be cool to allow it to function as a prequel or sidestory (that doesn't take away from the main game if it is not played). I remember LucasArts adventure game demos used to do this, and it was always cool playing them, as they were their own little unique mini-adventures.