Sweat pants all the way, in a dark colour (leaks, I'm afraid). Whatever you do, take stuff that fits you when you're 9 months pg as if you can't get back into your earlier maternity trousers you will feel sad and daft. Maternity tops are good because you don't snap back straight away but still have a bit of a deflated bump for a couple of days (well, I did) and then you find out that only so much of the bump was baby and its luggage and the rest unfortunately is cake and won't go away for a bit. Grr.
To be honest, I can't remember what I wore for leaving hospital with DS. Anything that still fit, I think: I was humongous. And DD was born at home so it wasn't an issue (but I packed black trackies in my emergency transfer bag).
It also depends on whether you are having a c-section and/or plan on breastfeeding.
Basically, I would take things that are as comfy as pyjamas but that look okay for wearing outside and concentrate on making sure your hair looks good as that won't have any practical implications for your comfort and can make you feel better if you're a bit sore and tired. I had a makeup bag and travel hairdryer in my hospital bag, with shitloads of concealer a priority.
Big scarves are good, too, for lifting a tshirt and sweatpants out of pyjama territory.
Take comfort in the fact that nobody else on the ward will be in cocktail dresses yet!
Oh, and one of the best tips I ever got was to stay in pyjamas or sweatpants as long as possible because as soon as people visiting the baby see you in normal clothes they will stop helping and will start expecting you to function as normal e.g. bringing them cups of tea and making lunch. Wear your pjs and you are delicate and only just out of bed and need cups of tea making for you.