Rec request: recreational math for kids

My friend’s 7yo is already a math-enjoyer—he loved the book “Math for Curious Kids” and keeps wanting to find out more. I’ve found a few things that have been hits ( a graph theory zine for kids, a group theory coloring book ), but I’m always on the look out for more. If anyone has some other fun math books for kids, especially that have some game/problem-solving/recreational aspect, I’d love to recommend more things for him!