She isn't really nervous about the surgery yet either- but I know she will be! She's actually excited about the idea that she will have a bed that she can adjust and a 'menu' of food options given to her daily where she can choose (maybe not the first few days, but after)...I don't think that she'll be on anything more than liquids for a few days. She's excited about getting to stay and having everyone wait on her- I don't think she fully understands how horrible she'll probably be feeling. I've told her that she'll feel 'sick' and may have a stomach ache, but she doesn't really understand.
I think what you have told her is just right, and hopefully she won't be as miserable as you think. Pain management is HUGE with Joint Commission now, and I can only imagine a place like Mayo would be on the cutting edge of keeping patients comfortable. Also, if she isn't *expecting* to feel terrible, it's less likely that she will, if that makes sense.
Taking her to buy some special things to take with her is a great plan. Things like crayons and coloring books are always good too. She can bring her favorite DVDs, but the hospital should have a library where you can check them out as well. Find out if their Child Life program does a pre-operative tour for kids. If so, that would be great for her. They explain things like the mask for the anethesia in ways a child can understand. She can even start thinking about what flavor she would like. My dd had minor surgery and was very excited to choose her flavor. They had everything from chocolate to Dr. Pepper!
I'll be thinking of you and praying for you on the 16th!
ETA - selecting food from the menu is also fun for kids, but in case they don't have anything on there she likes, she can possibly choose something like a hamburger, chicken fingers, pizza, or fries, as long as that's on her diet, even if it's not on the menu.

You can also always order things like pudding and jello, that are great for when she first starts eating.