Why do you need light daypack?. Doesn’t matter how you are traveling – backpacking with large backpack or traveling with big suitcase – you most likely will need daypacks for daily use. There is large variety of daysacks, but current trend is packable lightweight daysacks. They are so great. Usually weight only 200-300g (7-9 oz) and are very compact. You can keep it packed away in your bigger bag until you need it.
I am using Lifeventure daysack. It is 18l daysack and when it is packed, it is only 200 x 190 x 70 mm (8 x 7.5 x 3) dimensions.
I am not sure can you still buy such bag. But now there are many similar bags and they are much cheaper comparing with the price I paid. For 25-40$US you can get decent daysack and your back will be thankful for such light burden.