This made me chuckle! My husband has never carried a bag/backpack but since we live in a very car-centric area (with no public transit), he always kept the car fully stocked with various supplies.
He's the one who has the full first-aid kit and medicines (e.g., not just pain-relievers, but also hydrocortisone cream for bug bites, children's Pepto for tummy aches, and Benadryl for allergy attacks). When they were younger and potty-training, he even had a Bjorn potty chair for emergencies. There were usually toys or books in the car, too.
He also has extra water, non-perishable snacks, a change of clothes for the adults (we learned that lesson after our youngest spit up all over him while we were on a day trip 4 hours from home), and typical emergency supplies in a car (e.g., micro-starter for a dead battery, emergency flares, emergency blanket, etc.).
Anyhow, having a fully stocked car meant I just kept diapers/baby wipes/changing pad, a change of clothes, a burp cloth, and snacks/water for me (as a nursing mom) in my giant diaper bag. Having a lightly packed tote saved my back many times!