Make sure that everyone knows your reaction to the standard painkiller because tramadol and endone seem to be the go to. Also, if you have nausea problems with anaesthetic make sure the anaesthetist knows.
I took loose soft short sleeve pyjamas that buttoned through the front. I didn't find that I was much at focussing on books so a couple of movies to watch on my laptop (with headphones). For me, constipation was a HUGE issue post-surgery and the coloxyl didn't really do much so make sure you get on top of it before you leave hospital. If you've got a fridge in your room, something nice to drink for when you're sick of lemon cordial, or nice teabags if you've got access to a kettle. And make sure you pack some face wipes - my face felt perpetually greasy in hospital and there's only so many times you can wash it.
Make sure that the breast care nurse goes through your exercises thoroughly before you leave.
Trickiest thing to do at home is getting things on and off the clothesline.