like jacq said, almost all dryer sheets are made with tallow, which is just another word for lard or animal fat. you can do several things. either make sure they're never in the cat's reach, or you can buy vegan-approved sheets that are made with plant products. there are some that are not egregiously expensive. they still might not be good for your cat, but if there's no tallow then the cat probably won't be so attracted to them. the tallow is what attracts them, and that wouldn't be so bad in and of itself, it's the other chemicals that are hazardous.
it is extremely hard to be a good vegan, because there are hidden animal products in so many things you would never suspect. and if it isn't the animal products, it's petroleum products and hidden toxic chemicals.
(just incidentally: they make the white part of oreos with beef fat, also the filling in twinkies. so if you see your cat going nuts for either, that's why.)
hair ties, dental floss and threaded needles have to be kept away from cats. actually any long string that you aren't on the other end of because the barbs on a cat's tongue prevent it from spitting it out once they start swallowing it so down the entire thing goes. which probably doesn't sound bad, except that if it gets wrapped around the intestine (and it often does) you get bowel obstruction, very expensive surgeries, and often, death. so you don't want to accidentally kill your cat by letting it play with something that looks like a lot of fun.
you can still let them play with strings, just be sure you're holding on to it and never let your cat play with strings unattended. you'd be surprised at what they can swallow.
