Anything which needs to be connected to and electric supply (e.g. an ipod charger) can be tricky, because places can be very strict about electrical saftey. Both of the hospitals I worked in required all electrical equipment to be saftey tested by the estates department before it was used on the ward. This sometimes took a while because clinical equipment was given priority over patient's personal possessions. There are only a limited number of electric sockets available for each bedspace and clinic equipment would have priority for these. Battery operated items are much better.
You also need to consider security, because you do sometimes get opportunistic thefts and the hospital is very unlikely to cover and loss or damage to personal possessions.
It's also woth considering what your sister is having done and how this might affect what she feels able to do afterwards.

For example, headphones can be problematic after ear surgery (or any brain surgery where the ear is used for access), reading might be difficult following ophthalmic procedures, and spinal surgery often requires a period of flat bedrest (not everyone is able to read whilst lying down!).
Hope you find something she likes.