Nothing, really.
I honestly tried to think of something, but everything has value to someone.
One example some people might suggest could be, “sitting around smoking weed playing video games”. But to the person playing, it is likely very relaxing for them, so that’s not really a waste of time.
My current hobby is spinning wool. Many people would consider this a waste of time, in fact one person asked me, “it’s so much cheaper and quicker to go out and buy wool, why would you waste your time making it?” But it’s not a waste of time at all to me, I enjoy every step of the process, as well as the ability to create a beautiful and unique wool from a raw sheep fleece. I also enjoy learning about these steps and viewing other people’s creations.
Even sitting around doing nothing has benefits. Meditation, anyone? Naps?
Oh, I thought of one! The biggest waste of time I can think of is complaining. Instead of complaining about something, why don’t you go out and change it? [And even complaining has benefits at times!]