I'm having my 3rd CS in 3 1/2 weeks. Big yes to the big knickers. I just bought 10 pairs of the biggest ones I have ever seen in BHS yesterday, and they were on BOGOHP, btw!
I have high waisted maternity PJs for night time once the catheter comes out, a nightie for while I am still in bed and before the catheter comes out, and soft, high waisted leggings and loose tunic tops for going home in / wearing on the ward (like Sidge I'm not ill and not going to wear PJs all day).
I also take a full make up kit for photos (after horrific photo experience with DD1!!!), and extra bath towels (as I am paranoid and don't use the same towel twice after a CS!!!!!). I've bought a couple of magazines too, as I don't resent being broken off reading a magazine as much as I do a book! Looooads of maternity pads and breast pads too!
At our hospital you can take your own music to play during the op, so you might want to find out what they can play (CD / memory stick etc) and take soemthing along.
Not related to bag packing, but I'd also say to get up and out of bed as soon as you feel able (obviously with the agreement and help of the mws!!), and to get the catheter taken out asap. Use the call buzzer as much as you need to, and don't feel bad about it. Ask the mws for help with whatever you need (even things like bathing the baby for you if you want), as you will need more help at first than with a VB. If you are in pain, tell the mws and ask for pain relief. Oh, and it is completely amazing how much difference a shower makes once they say you can have one - it can be a bit of a nerve-wracking thought, but both times I've been like a new woman afterwards (one of the mws didn't recognise me afterwards last time!).
Good luck, and enjoy meeting your baby :)