I am forever finding lonely puzzle pieces from puzzle boxes my daughter has collected over the years. I would store them in plastic baggies always thinking that some day the lonely miscellaneous pieces would be needed to once again finish a puzzle. Apparently I thought wrong. I now have several baggies with many various unclaimed puzzle pieces.
So here is one idea for lonely puzzle pieces that you might happen to have around your home too--
Use them to create a picture frame, put your child's picture in it, and on one part of the frame, paint the words "Love you to Pieces!." This should make any Grandparent smile when they get it as a gift!
You can glue the puzzle pieces to a wood type frame, or make your own frame using poster board. Just cut a piece larger than the picture. Glue the picture in the middle of the poster board, and your child can glue the puzzle pieces all around the border. Over-lapping the pieces makes a great look.