I'll jump on the gift card bandwagon. Some new parents are very picky about things for their baby. They may want a particular brand of diaper or whatever. To some, a diaper is a diaper, but to a new parent, that is not necessarily the case.
Diapers or when the baby is a little older, the offer of a free night baby-sitting (depending on your child care abilities).
Diapers are a can't miss. Gift cards for formula would be good too. Or onesies...the quintessential outfit for babies under 12mnths. I'd be lost without 'em.