Water bottle. My giant Nalgene is in like 87% of the hospital pictures
A BreastFriend (I used both a BF and a Bobby...BF wins for the first few weeks!!)
Nursing bras or tanks, though really, after pushing a baby out, your modesty is gone.
Camera + charger
iPad/iPhone + charger
Your own pillow in a colored case (so the hospital doesn't think it's theirs).
A bath sheet. Hospital towels are itty bitty!
Makeup, contact stuff, toiletries....
A comfy going home outfit. I wore a maxi dress and cardigan.
Any paperwork your hospital requires
A "birth plan" if you didn't already give it to your OB
Cozy loose flanel pants...I wanted NO extra pressure down there
A few cute nightgowns
If your doctor thinks you might have a peanut, bring one premie outfit. E didn't fill out her newborn clothes till 2.5 months.
A going home outfit
...and that was 48 hours after giving birth. Yikes.
We have a Britax Chaperone and the seat came with the "premie" insert already installed. Elizabeth was 6.2 when we left the hospital and the insert is suggested for use up to 11 pounds. NO. WAY. We physically couldn't get her in the carseat and buckled. We looked like total idiots to the nurses. Eventually we figured that we had to remove the insert. So be aware.
You will want to wear the mesh panties and pads they give you. And the ice packs...heavenly. I brought grannie panties with, but didn't use them till we got home.
Most of the time I was topless and cuddling a diapered Elizabeth under the covers. We had as much skin-to-skin contact as possible. B did too, sometimes I would nap and he would take his shirt off and cuddle with her.