The thought experiments Kaku talked about made me imagine the possibilities of the universe. It's a great book to get kids interested in science since it explains the intuitions without being stuck in the precise details.
Intuitions of what? Well, intuitions of space, time, and ultimately spacetime of more than three dimensions. It has nice thought experiments to help develop intuitions of what it's like to have more than three dimensions. Kaku also explains the intuitions around string theory, which is fascinating as well (even if it turns out that string theory is wrong, it's still fun to imagine the universe in a different way).
A great book, especially for kids. Highly recommended!