Take straws and a sports bottle (drinking out a cup isnt always practical in labour) sports drinks like Lucozade are useful if your labour is long, helpful to keep up energy. You can also get glucose tablets.
A couple of those puppy training pads (on top of the bag) for the car on the way in.
Comfy/pj shorts (post natal wards are hot)
Dry shampoo (IME the showers are alright but you don't wanna spend too long in there)
Front opening nighties if you are planning on bf.
More maternity pads than you think you'll need (And then more still)
Hair bands
Really good snacks, things that are comforting not just the odd granola bar.
Finally, lay out everything you'll take and get your birthing partner (DP or otherwise) to pack it so they know where everything is.
To be honest though, I packed loads and used nothing but a water bottle until baby arrived. I had bought nighties to give birth in and couldn't have cared less at that point. Whatever you go to the hospital in, make it easy to remove for skin to skin. I have a tight sports bra on and it was a nightmare to take off!