Nonograms, also known as Picross, hide a picture inside a grid of numbers. Color Nonogram Puzzle 2 adds color to the idea, and each solved grid turns into a small piece of pixel art. There are hundreds of levels, and the play is calm and stress-free.
A sensible way to start is with the lines that have the largest numbers, because they leave very little room for doubt. Sudoku fans will like the logic here, and Sudoku Classic is the natural companion. The first game, Color Nonogram Puzzle, is worth a look too.