U1 is possibly my favorite adventure of all time. I've run it in every edition of D&D and two editions of GURPS. The basic plot is linear in the sense that you've got House, Cave, Ship, but other than that I've found that it never plays the same way twice. Some groups go in the back door and head straight down to the basement. Some of them spend a full session exploring the house. Sometimes Ned enters the picture. Other times he doesn't. One group decided to explore the environs around the house more, including the cliffs, and discovered the sea cave before they even set foot inside the house.
My most recent group captured Sanbalet in their first encounter with him in the basement and then attempted to use him to gain access to the caves without setting off the alarm. He, naturally, tricked them into heading into the slime, and managed to escape in the ensuing chaos. This led to a very satisfying confrontation in the sea cave as the smugglers tried to escape.
I've scaled it up and down for different levels of play. Last year I ran it with the
Dungeon Fantasy Roleplaying Game (GURPS) where the PCs start closer to what might be fifth level in D&D. It still worked great. I just beefed things up a bit and improvised.
I've been surprised at how satisfying players find it to resolve the mystery, even one as simple and predictable as this one.
Watching Scooby-Doo might be boring for many adults, but
experiencing an episode seems to awaken one's inner child (in the best possible way).