I packed mine at about 34 weeks initially then added to it as I went along. I actually packed 3 bags!
A labour bag - rucksack, with clothes, maternity pads, fan, snacks etc.
Baby's bag - changing bag, with all her clothes, nappies, muslins etc.
Overnight bag - mini suitcase, with pjs, undies, dressing gown etc.
I would recommend a lightweight dressing gown, slippers, dark coloured towel in particular.
For labour, I found a handheld fan to be invaluable as I was so hot throughout - it was the only thing that I actually took out of my labour bag!
My OH also packed a rucksack with food, toothbrush and a change of clothes.
I would also recommend getting those ice pack things in the freezer at home in case you need stitches.