I was amazed to see your post ggmarion because I have been thinking exactly the same thing lately, although not for myself, but for my DH. Hopefully it will never be needed, but he does have a long term condition, and as he never wears anything to bed at home that may not go down too well in the Hospital
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I thought to pack PJ's,underwear, some light slippers, soap, flannel, hand towel,light dressing gown toothpaste and brush, a couple of books, a little bag of small change. That would be a good start I think. Phone, medication and anything else could be thrown in at the last minute.
I have not mentioned this to him as I know if I suggest it, or come home with a pair of PJ's he will say I am being morbid. I will just tell him to think how he would feel if he was packed off and all he had to wear was a Hospital Gown!