Questions from nothing.
Come up with riddles out of the blue, for no reason and do not fit them into the narrative. Then the players will puzzle longer over the task, and perhaps they will not solve it at all. And probably players will even forget about the quest's story while they are looking for an answer.
Demonstrate your knowledge.
Don't explain riddles and punish players who read less than 100 books a year. Check the players for knowledge of little-known facts because you need only the most erudite to complete your quest.
For initiates only.
Do you want only professional players to complete the quest? — Feel free to write the most ingenious questions, the answer to which will be "Pushkin".
Alzheimer's disease prevention.
Players must carefully follow and memorize all questions and quest narratives. Why not ask in the finale what happens if you combine pictures from the third and fourteenth questions?
Testing on smart people.
If you doubt how difficult it will be to take this or that question — send it to the most intelligent and erudite friend.
Don't check anything.
Factual errors, a comma put in the wrong place, or just a typo will make life harder for the players, and your quest will become even more difficult — but that's what we need, right?
My opinion is the law.
Make questions based on your own judgments and assessments because if the players do not know you, this is not a reason not to take a question - this is a reason to get to know you better.
Why come up with interesting questions and non-trivial riddles? After all, players do not go through the quest to answer cool puzzles and stretch their convolutions. It is better to ask them to reproduce the inscription on the building's plaque or to complete the well-known folk wisdom.