Kids grow and car seats expire, but what do people do with millions of used car seats? They try to recycle them but most end up in landfills. So maybe the materials degrade over time and may not be able to handle 60mph car crashes, but they still have function! They can be used in lieu of ridiculously expensive, and not always supportive enough, TumbleForms, Special Tomatoes. Car seats can also be used to adapt all the things you usually adapt with special tomatoes including swings, sleds, wagons, beach wheelchairs and consumer wheelchairs. I originally got this idea when seeing a Special Tomato used as a car seat. Car seats have awesome 5 point harnesses, attach to chairs pretty easily (may need an extra strap or 2), provide various degrees of support and come in fun styles! Best of all, they are super affordable, especially when crash safety isn't a priority.
Bonus: a lot of people try to sell used or expired car seats, which, yes can be sketchy, but if you're not going to use them in cars, it could be a great deal (obviously make sure everything relevant works) or maybe family and friends are looking to get rid of theirs.
Find a car seat that fits your needs: head support, lateral support, height, comfort, etc. to get started. I haven’t come across any resources about up-cycling old car seats specifically into adaptive seating, but there are some for toddler seating and adaptive swings.
I just stumbled across this blog today and it is incredible! This post has lots of other ideas, but includes swings and sleds!