My father-in-law enjoys Sudoku but was doing puzzles with a sharpie because his vision is so bad, he can't see pencil marks. This was my solution. The Crayola dry-erase crayon (available at Michael's, JoAnn's, Office Depot or Staples) makes thick lines that are easily erased from the sheet protector using kleenex or paper towel and doesn't need to be capped, so no worries about a marker drying out. Folks with normal vision who remember to put caps back on dry-erase markers can use regular dry-erase markers if they wish. This set-up is obviously suitable for any other template you'd like to be able to erase or re-use, such as a crossword puzzle, calendar or checklist. Insert a blank piece of paper and you've got an inexpensive DIY whiteboard.
Rockingd made it!