okay, even if they have everything for this stage, kids change so quickly. In the next 8 weeks the new baby may be able to roll over, start teething, and some kids will be sitting before 6-months. They grow exceptionally fast. They fly through diapers (and diaper sizes).
My kids never used pacifiers, but if this baby does, finding that "type" and getting many extras would be helpful. Teething toys, developmental toys, the next size in blanket sleepers, more developmental toys and rattles, baby carrier (get a good one!), those mirrors for seeing what baby's doing in the back seat, a crib mirror or crib aquarium, formula, bottles, cloth diapers (for burping), gift certs for photography or even film developing, a good play pen with bassinet/changing table, a high chair (many folks think they won't use this at 3-months, and it's not necessary, but it is so nice to have another safe place to put baby), an electric swing,
and because it's just around the corner: feeding items, spoons, storage containers, etc.
Some things I didn't know as a first time mom, or even as a second time mom that I wished I'd known:
the high chair really is helpful at an early age
a good stroller is worth every penny (consider a maclaren or peg pergo even though the cost will make you gasp. the resale value is great and steering really is an issue)
An excellent baby carrier can help you take a baby anywhere without all the contraptions of a car seat and travel system
babies outgrow clothing so fast that having too many precious outfits means that some just won't get worn. -- and try them on baby because all 6month sizes are not created equal!
have fun being auntie!