Please be careful with things like string or yarn. If the cat swallows it, and many of them try to do that, it can lead to a painful death as the string or yarn can wrap around the intestines.
However, I agree that there are plenty of safe things around a house to provide entertainment and exercise for your pet.
We had to quit adding tinsel to the tree each Christmas because our cat kept playing with them and trying to eat them. ...we knew that would not end (ha) well. My beloved cat, now waiting for me at the gate of heaven, loved laying under the Christmas tree. We had to leave a chubby area where no gifts could go, just to give him a good spot to nap.