We usually give a few religious books (board books are especially good at that age and there are a lot of cute ones out there), some sort of religious gift to commemorate the day (a "Christening/Baptism" picture frame or cross), and if iti is someone we know well, some sort of monetary gift.
For a 15 month old, I would highly recommend books. You can usually find children's religious books at any bookstore - you don't need to go to a religious store.