Here’s a really good list: 11 Things to Pack When Heading to the Emergency Room.

A few thoughts/additions:

• take your smartphone. Much of this information can be found there. Indeed, you can put much of this information in a central, easy to find place on your phone, so when you are at the hospital and at admittance, you can easily share it with the hospital. Plus you may use your phone to order Ubers, pay for things, update work, etc.
• take a smartphone charger. Your smartphone is your contact to the outside world. It’s also a reliever of boredom while you wait for treatment. You don’t want it to die on you while you are in a waiting room.
• take your wallet. Things not on your smartphone are there.
• take some snacks (e.g. granola bars) and water if you can. You may want to have some sustenance to keep you going if you are waiting for awhile. Don’t assume your trip will be brief or you can find food there.
• take something to listen to sounds on your phone. It can help you stay calm and relaxed when you are waiting.
• write down the timeline of events that made you decide to come to the ER. Do this either on your phone or on paper. It will help you when you have to repeat things and you and you are too tired or ill to tell them.