I didn't put it in my registry, but my mommaroo swing is my favorite thing. I didn't get any diapers, and got a ton at my shower for my first daughter, so that was a little unexpected. Gift cards are also awesome. I also wish I'd gotten a stroller, but I'm sure you already have that on there. Other things people sometimes forget are grooming kits, towels, and hangers.
I want a mamaroo but they are pretty expensive
A swing!! A plug in the wall swing is a must.saves on batteries and the babies love them.
Honestly, no one ever looked at my registry with my son so I'm not wasting my time with this one, but a swaddling blanket, definitely!! My son LOVED it!
OH and I'm 21+3 and having a baby girl too!! :):)