I don't scrapbook much anymore so I'm not sure what products are available, but there used to be smaller-sized, inexpensive albums with comb binding, similar to a chipboard book. They did not have protective covers on the pages, but they were acid free and worked well for small albums (I used one to scrapbook a vacation). Not sure if something like that is available still, but the smaller size is nice because it doesn't require as much time and effort to do layouts and it doesn't need much embellishing. If you can use coupons at the big stores, you might be able to get something like that pretty cheap.
I would imagine they would want to include sonogram pictures (if they have them), maybe a pregnant "belly" picture, pictures of other family members, pictures of the baby's birth, etc. So some of the pages may need to be left blank (or completed except for the pictures) until after the baby arrives. It would be nice to leave some pages for journaling so they could write about their pregnancy and the birth in the book if they want to. I think you could make something very nice in 4 weeks' time, especially if some of the pages are to be filled in after the birth. If you could give them a little packet of acid-free adhesives and an acid-free pen to finish the book after the birth, that would be helpful (and something maybe your local store would donate).
What a wonderful idea you've shared, and what a good heart you have!