1) Elder care. It's no surprise the childless siblings bear the bulk of the elder care responsibilities in the U.S.A. The exception is when parents invite Mom to move in so she can babysit and do the laundry.
2) Volunteering. The majority of the folks I interviewed had volunteered for at least one non-profit, including Big Brothers/Big Sisters, animal shelters, equine therapy programs, youth advocacy, volunteer coaching and mentoring, and other charities they were passionate about.
3) Second jobs. Contrary to popular belief some childfree folks need to work a second job to pay school debt or just plain pay the rent. Others are entrepreneurs with start ups or artists or writers and their creative or entrepreneurial pursuits can't pay all the bills.
4) Childcare. Yes, some childfree persons actually help care for other people's kids. I interviewed folks that were temporary guardians for kids whose parents couldn't handle them or care for them. Some took nieces and nephews on vacations and college search trips because the parents couldn't afford to travel or take off work.
5) Recuperating from illness or injury, or caring for pets. Many childfree people complain that when parents ask for time off for a school recital or a snow day they are quickly granted the time off but when a childfree person asks for time off to go to physical therapy or a vet appointment they are denied. This is wrong!
Jenny Noyes, a writer from Australia, make a strong case for equal opportunity for Flex Time in her article titled "The childfree deserve workplace flexibility too" with the astute observation that "having it all" means different things to different people.
I know from my interviews with childfree folks that they have many interests, pursuits, obligations,and responsibilities outside of work and a flexible schedule would be a welcome benefit for every working person. Flex time is a benefit that everyone appreciates and it should be offered to all. Ideally, everyone should be expected to work the number of hours they are paid for, on a schedule that allows for a life outside of work. What you do outside of your work is your own business, unless the cops or social services are knocking on your door.