This is not new idea but I'd like to tell you about it.
This is my friend's idea. Once Max (my friend) took a Puzzle game of 5 000 parts and presented it to his girlfriend. I should say that she loves to play this game and she has many puzzles in her room.

Max's idea was to collect the puzzle and then on other side if the picture to write "I love you so much!", You can write whatever you want on it, it's up to you. ;)

When his girlfriend had collected the puzzle she read the message and she was so happy that she took a taxi, came to my fried and say many good things (it's private that's why I don't want to spread her words)

It was super!