This is a fun and affordable party favor idea: pocket mirrors! For under $20 you can order 10 custom mirrors from the WistfulSupplies Etsy shop. I always find myself needing a mirror, especially since our car doesn't have one on the passenger side visor.
Since you get to design them yourself, the possibilities are endless. It would be cute to use an old wedding photo for a relative's big anniversary party or a design that subtly matches your invitations. Or, you can order pre-designed mirrors from Pink Loves Brown.