As a mom of two girls, (4 1/2 years & 7 months) I can't help you with my personal experiences except to say that both my girls love(ed) their swings, jumparoos, and bumbo chairs. As a mom, I cannot live without the Dr. Brown bottles, floppy seats, and their Britax carseats.
Pretty much all of the great baby stuff can be used by both boys and girls, provided that you have the appropriate color or pattern for a boy or girl. Not sure if the baby's daddy would be happy to see his son in a pink stroller!
My friends that have boys do tell me all about getting peed on if they aren't careful during the diaper change...so maybe a "must have" for boys only would be a Pee Pee Tee Pee...which is a special cover that you put into place that prevents this from happening...
Hope this helps and congrats to your cousin who is more like a sister and her soon-to-be baby boy that will be more like your nephew.