It really depends on a lot of things -- how long you anticipate being in the hospital, if you have a c-section or vaginal birth... etc
You will need things for the baby of course, most hospitals require that you have a carseat installed (and some have someone come check and make sure it's in there properly!); if you want you can pack things like a ring-sling or wrap/mei-tai to wear the baby when you walk the hallway between your room and the nursery or food-court...
You'll want PJs, and likely want something to freshen up with (shampoo/conditioner, soap, etc) -- and it depends on your hospital and all that if they provide towels for you or you have to bring your own.
I am still packing my bags, I keep thinking of things I did not include just yet, and guess what -- I have NOTHING packed for the baby yet.
I'm planning on about 5-7 days in the hospital (as that's how long I was in last time) so I need things like PJs, pads, socks, house shoes, housecoat, clothes to wear home (not sure what I will wear home, mostly due to not knowing if it will be natural or c-section), hairbrush needs packing, some makeup, deoderant (more for going home than not - as they reccommend no fragrances around baby), mud mask, ponytail holders... I need to pack snacks, and I've not had time to do that, and figure out something as far as reading or "free time" between the time I'll be awake and baby is asleep... we won't be having visitors as much as last time, I don't think..
If you haven't already done a maternity ward tour, I'd highly reccommend it - and ask then what you need bring when you come.