Why not give away Sudoku puzzles? They're totally fun esp. for those who love challenges. Pick easier to solve ones for younger children and teens.
Then there are Picture Puzzles. :) They're pretty fun and challenging to solve.
I've always liked Jigsaw Puzzles. I'm sure kids do too! Find age-appropriate puzzle toys that children will like and cherish.
Last but not the least, Crossword Puzzles. My late dad has a fascination for them. He used to answer the one from the daily tabloid. ^_^