What to write

So my husbands cousin is having a baby and I don’t really know her too well but she’s super nice and I’m extremely happy for her and I’m hoping her baby and my son will be close because they will be close in age. Anyway they’re doing the thing where you write in a book for the baby instead of a card and i don’t know what to write. Any suggestions for someone I don’t know too well? My husband also doesn’t know what to write lol