I really enjoy books that add other activities that I enjoy. I love puzzles so I've been particularly drawn to books that have puzzles. I mean real puzzles.
The first book I encountered was Maze, by Christopher Manson. It is a book that challenges you to work through the pages by following the numbers. If you figured it out and sent in the solution you were eligible to win $10,000. It is somewhat hard to describe, but someone created a website that takes you through the process. Wikipedia also gives a pretty good description as well as the solution.
The second book I received was Ruby by Colin Thompson. This book was more like an I Spy in which you solve the puzzle by looking for numbers and letters to make up the license plate of a car named Ruby. Finding the right information (the license number plus the answers to three questions) and sending it in gave you Ruby, a vintage Austin Seven Car. Information about the winner and the author can be found here.
The third book I acquired was A Treasure's Trove by Michael Stadther. This book was extremely popular as twelve valuable jewels were the prize and they were hidden in twelve separate areas in the United States. There used to be complete sites dedicated to the search for the jewels that seem to be no longer available. Once the "hunt" was over an official solution book was released.
Stadther has since released Secrets of the Alchemist Dar, another treasure hunting book boasting $2 million in jewels. According to what I can find on Wiki the book remains unsolved and the hunt seems to have been suspended.
I am not enough of a puzzler to have solved any of these books, but there was a lot of fun in trying.