one of my childhood favorite games is Little Big Adventure 2: Twinsen’s Odyssey and i first played it on a PC Gamer demo disc (the holy grail of magazine purchases to beg your parents for). It was a 3-part vertical slice of the game where if you transitioned from one area it would kick you to the next part of the demo. The first part lets you roam around the starting island during a rainstorm (the first act of the game) and i found it so lovely and atmospheric, thanks in large part to the very good music
if you enter any houses you get booted to the second part, which is weirdly enough a completely optional cave area that you enter to get protection magic in the full game. If you leave the cave you get sent to the final part, which is wayyy later when you’re in the molten core of an alien gas planet. I remember being really intrigued by the scope of the game as implied by the demo.
The other memorable demo from my youth was the one for the Neverhood, which is only notable because it has a unique music track not in the full game
