I got to thinking about like, Amazon. To an American in 1970, the equivalent would have been Sears, the retailer that sells literally everything though an enormous catalog. They had department stores you could visit in person and they'd publish a phone book thick catalog for mail order. And there was a time that Sears could have bought Amazon outright for chump change. I remember a Tim Allen stand-up routine where he talks about mail ordering a tool from Sears, and it arrives 6 to 8 weeks later. Shipping is now usually a matter of 1-4 days.

But no, the real answer is probably wireless headphones. Like if you were showing around a tech enthusiast from 1975 around my house, the thing that would probably break his brain the most is a pair of wireless earbuds.