I think there were no truly hard puzzles in TWP. Here is what a hard puzzle looks like: you arrive on an island and you see a wanted poster with your face on it. A guard looks at it, then looks at you, recognizes you, and jails you. You think the poster has already accomplished its task. Instead, the puzzle is that you have to stick the leaflet that Kate gave you, which has her face on it, on the poster. So she will be arrested when she gets on the island. But you had no particular reason to want her jailed [you only knew you wanted something that she has; but from that you would never infer that you need to jail her]. And you had no clue that the poster was an object to combine; it seemed part of the scenery and it looked like it had already accomplished its function. And you had no clue that the leaflet with Kate’s face was important either. In fact, it was important not for its usual function, but for a secondary property (what is depicted on it). I think that’s the best puzzle ever.