make the heart out of paper and write the "save the date" on it, both of your initials and the date super simply, and then the actual wedding invitation with it! then they can put the real invite somewhere nice and the little heart paper on the fridge!
DIY: Ask your photographer to do this! It's a long exposure shot with sparklers. All they had to do was stand there very still and someone else ran around them with a sparkler. It's like a fairy tale! Long exposure shot with sparklers