Everything you need for baby and nothing you don't
Let me be 100% clear: for a baby shower, you should buy off the registry. Period.
I can’t stress enough how you should NOT take matters into your own hands and buy the expectant mama what you think she needs vs. what she has painstakingly decided she needs. I know you have brilliant ideas, impeccable taste and maybe even your own recent experience to draw on, but you should still get her something off her registry. It’s a sign of respect to the trouble she went to in creating her own very personal idea of what she wants for her baby.
If you’ve had a bridal or baby shower yourself, you know what I’m talking about: do not be her Aunt Myrtle, showing up with the serving dish built especially for eggplant that she believes every kitchen needs and that you will take directly to the Salvation Army.
Also, it is 100% irrelevant if you think your friend is wrong about that diaper disposal unit she chose for her nursery. Offer your two cents about a better one gently over email if you must, but if that one you hate is still showing up on her registry in a week or two either get it for her or find something else. If you’re right, she’ll know soon enough + there’ll be no damage to your friendship in the process ’cause you got her something she didn’t want!
There. I’m done. I’m just trying to set you and Mama up for success.
Of course, once you’ve hit your mark and gotten her something she wants, there’s no restriction on adding in that book your baby loved, that onesie you couldn’t resist or any other amazingly cuddly items you couldn’t not buy. “You totally, 100% must have this even though you don’t realize it yet!” items are also always welcome as additions. Or as, “Yay! Your baby arrived!” presents. That’s what I do. It calms down my, “I’m helping!” gene.