Rebjane,
First of all, I truly hope your daughter does not have to go in the hospital! My two daughters are 'frequent flyers' at our hospital, I'm afraid. They're 13 and 18 now, and have been in a lot since about your daughter's age. The suggestions people gave above are all great. I particularly like bringing the noise machine and coffee maker, as well as my daughters' favorite crafts/stuffed animals/blankets/books/video games/washable slippers, etc. I know this may sound silly, but I usually try to buy them something new when they have to go in--fun pajamas, slippers, hair ties, nail polish, fun message t-shirt, or something to make them feel special (5 Below has great and inexpensive ideas). Practically, I bring extra plates/cups/cutlery too in addition to survival snacks we both like. Christine talked about vitamins tasting like barf--how about that hospital food! I also have one of those plastic basket type carriers for the bathroom to put shower stuff in (shampoo/cond/soap/razor, etc. so stuff doesn't fall on the icky floor). I've forgotten to bring Ibuprofen for me in the past. It was awful when I couldn't get anything for a bad headache! Just think what you use routinely and would like to have with you to make you both more comfortable.
I do want to make a comment about meds though. I always bring a supply of all the important meds just in case (enzymes and bronchodilators are the most important). One of my daughters has really reactive airways and can't miss treatments when she's sick. For recent hospitalizations, she was put on nebs every 2 hours to start with. On admission day, it may take until the next shift to get the respiratory treatments started, and that's just too long to wait. So I bring my own meds/neb cups/tubing/machine (if I can't access the wall connections). If my daughter's having an asthmatic episode, I'll start a neb and tell the nurse. Sometimes the respiratory therapists run late. We'll have them paged and if my daughter's cough escalates too much, I'll give her a neb. I always tell the nurse what's going on so she can chart the meds. It's not ideal to do this, but sometimes necessary.
Being in the hospital is stressful, with worrying about your daughter getting better, broken sleep, bad food, etc.; but it can also be great bonding time with your daughter. That's the silver lining for me. I have very fond memories, even with the drama at times, of being in the hospital with them.
Best to you and your daughter!
Sherri