My recent project is a 'fill-in-activity' book for babies (originally my little niece-to-be-born), their parents and whole families. The book has got a subtle storyline and once you fill in all the details you've got a lovely narrative to read to your baby when he or she gets a bit older. When they are big enough, they can have their own go as there is plenty of space to collage photos, draw pictures and write your own stories. The whole family can (and should!) engage.
I have also added little pop-up surprises, guessing games etc. to make the book slightly more interactive and fun. Hope the little one likes it.