Anybody who’s played Sudoku probably hates the eventual drudgery (at least with the hardest puzzles) of writing in lots of little numbers. Either the ones that COULD be in that square, or the ones that CAN’T. Suppose instead you had a rubber stamp with tiny numbers. You’d lightly stamp the squares with fast-drying erasable ink, so fast-drying you could immediately wipe away the numbers you didn’t want. Or suppose the no stamp could make clear enough tiny numbers. Instead the stamp could leave dots whose positions corresponded to numbers. The top left dot would be 1, the center middle 5, the bottom right 9. Would be even easier to erase. Maybe better yet would be a book or newspaper puzzle that came with preprinted number sets in the squares. Again, the numbers would be easy to erase.
Another puzzle idea: Puzzles Cereal