A cleaning voucher. When her babies are at home and she's flat-out caring for them a cleaner will be a big help. I think there is a company called Buzzy Bees but if they don't worl locally trere are lots of other cleaning companies you could contact (or dial-an-angel might do cleaning).
Depending on your budget you can also buy fingerprint charms (but you would need 2), a post-natal massage voucher, a photography voucher (or take some photos yourself and get the best ones enlarged and framed), some home cooked meals are always welcome in hospital if she's not on dietary restrictions or fill her freezer for when she gets home.