I am putting a cotton pad and a bottle of essential oil in mine. This oil is a Christmas mix of cinnamon, cloves and ginger. You can also add orange for Christmas. If you make the hearts from other kinds of fabric, they can hang all year round and have any fragrance you like.
Hang them in a window or doorway, or somewhere there is a draught and the fragrance spreads.
Remember to use the cotton pad. Some oils can discolour fabric. (Guess how I know!)