Universal travel adapter (Type A, C and I)
various makers
China uses three plug types and you will meet all three, sometimes in one hotel room: flat two-pin Type A, round two-pin Type C and the angled three-pin Type I. Voltage is 220V, so check that anything you bring is rated for it — most modern chargers are, most hair appliances are not. Buy one with several USB ports and you will cover a phone, a power bank and a pair of earbuds from a single socket, which matters in an older hotel with one outlet.
What it does not do: An adapter changes the plug shape, not the voltage. It will not protect a 110V device, and the cheap ones make poor contact with worn Chinese sockets. This is also the easiest item on the list to buy after you arrive.
Free here first: Free hotel check-in phrases