So many great suggestions here, I have little left to add.
People have mentioned paper and writing implements to jot down questions. I suggest a notebook to keep track of everything so when you look back you will remember all that happened and that you went through. I meant to do that but the notebook didn't get packed and my husband couldn't find it at home. Of course I figured I would remember everything but I did not.

As @gaylea1 mentioned, a flashlight. I always pack a flashlight whenever I go away.

Also, a lip balm. I found my lips got very dry in the hospital. Thankfully they did have some there and gave me one but it took about a day and requesting it a few times before they actually did. Also, if you prefer something other than plain hospital water ask whoever is visiting you to bring something like Hint, or some sparkling water type of beverages.

Definitely something warmish to throw over your upper body. I have a light fleece jacket and it was a perfect weight and could easily be left on in bed when I was sleeping. I found out from previous hospital stays that I tend to get cold in hospitals.

I like to have slippers too for walking around. Those socks with sticky pads on the bottom just don't do it for me. Unfortunately, I had my expensive UGG slippers with me for one hospital stay and when they brought me down for an endoscopy someone took them off my feet and I never saw them again so don't bring anything too expensive that you value. They were brand new, so I definitely think someone stole them. After that I brought inexpensive slippers.

I can't think of anything else right now, most everything is covered, plus they do give you the essentials in the hospital so if you don't mind using their basic brands you are pretty well set.
JK