Device to "freshen" an unventilated bathroom?
Try putting a small dehumidifier in the room, which will cut down on musty smells.

Also if this bathroom is used infrequently, be sure the flush the toilet periodically, and run the faucet.  Stagnant water in the toilet will stink, and the drain trap will dry out, leading to sewer gas smells.  This will be especially true with a dehumidifier, which will dry things out.