They prefer an empty stomach/bladder so no food or drink before - obviously they make an exception for an emergency where the risk of vomiting outweighs the risk of not doing the section.
Things to pack? Huge knickers, huuuuuuuuuge ones, real granny pants. I got a five pack of belly-button huggers from Matalan for a couple of quid and they were amazing, they were nowhere near the incision, big enough for the maternity pads, and cheap enough to just bin afterwards.
Peppermint tea, Wind-eze, or something similar. My MW recommended half a co-codomol washed down with a shot of peppermint essence. That trapped wind afterwards is awful.
Something to give you a bit of comfort or a bit of a lift. I wasn't expecting a CS (I went in to have my breech baby turned, had DD by EMCS half an hour later) but I had some lovely fruity showergels and nice moisturiser in my hospital bag. Such a small thing but it brightened my shower up the following morning and perked me up a bit.
Lip balm, hospitals chap lips, I don't know why but they do.
Plenty of sleepsuits, vests, muslins, bibs, etc for the baby. I don't know about anyone else but DD was really mucousy after her birth (it's supposedly something to do with the gunk not being squeezed out of CS babies like it is with VB babies), she was sick loads the first 48hrs after and needed changing frequently.
Flip-flops for the shower and shuffling around the ward in. Nice warm socks for sitting around in.
Camera, we were allowed to take ours into theatre for those first photos.
Book or magazine. I had to stay in bed overnight, CS was at 11:30am and they had the IV fluids in until 7am the next morning (I had low BP) so I was stuck in bed and bored.