I never really bothered buying a proper 'buggy bag' as it was so easy going out with a baby! He was a breastfed, content baby - no need to carry more than a couple of nappies, wipes and a tiny change.
But now his bag has all the above (in bigger size!), plus food, drinks, medications that have to be carried with him at all times, often little toys.
The backpack I normally use is now SO heavy and bulky.
What do you use? Any good item out there that it's not heavy itself, adding to everything else?