I have a 3 and 5 year old and little play figures seem to be very popular at this age. They can be played with in a very unstructured way if she's wanting to play but feeling too tired to follow instructions or do anything that requires concentration or a steady hand.
Depending on what her interests are, the Disney favourites moments figures and playsets are lovely and a good size (similar to a playmobil figure in height) and have interchangeable dresses. We have one of the storybook playsets on that link and I can highly recommend it. They have similar figures for the Sofia the First series discounted in Sainsbury's at the moment.
Busy books are also brilliant for little play pieces, although these could get lost in the bedclothes as they are quite small, especially if she is feeling sleepy. ASDA direct have a selection for £5
Die cast cars would roll nicely across hospital bed trays. Hot wheels are great for funky designs (half price in Sainsbury's at the moment) and Siku do a lovely selection of real life vehicles. Might seem a strange gift for a girl, but my very girly 3 year old has just got into them and gets hours of fun from them.
Schleich animal figures are lovely and always well played with. They are currently 3 for 2 in ELC and I have a 25% off one-use voucher that I'd be happy to give you if you PM me.
A pair of binoculars could be fun for spying on the nurses looking out of the window from her bed.
How horribly helpless you must feel. I hope she soon returns to that bouncy little girl.