Sudoku,in pencil.

Sudoku books by Kenji Ninefold for people who leave a square empty until they know exactly what belongs there.

Which level fits you?

Medium: If you’re comfortable checking rows, columns and boxes for a missing digit, start here. America 2027 gives you 1,000 grids to work through at that pace.
Hard: Do you pencil two or three possible numbers into a square? These books give you a reason to keep those notes. Start with Level 1. Each volume asks more of you than the last.
Expert: Suppose this square holds a 4. What would that force in the next square? If following that trail is the part you enjoy, try Expert Level 1. There are three levels in the series.