Most of us have, at one time or another, arrived in a new city too early to check in at hotel or Airbnb, and with no desire to drag the bags around until the afternoon. Or, we've checked out by 11 am, and need a place to leave the luggage until it's time to move on.
My easy solution used to be the 'Left Luggage' service that was in nearly every city's main train station. But, over the past 20 years and especially the last 10, fewer stations have the service, or have it at impossible prices (at London Bridge Station in May, the rates were astronomical).
And so, more and more I've come to rely on the 'third-party' storage operators who have established themselves not only at the station, but sometimes in other parts of cities, especially in Europe.
They are generally reasonable in price, and charge by the day or half-day. In many cases, they are banks of lockers, operated by an attendant or a machine; you pay upfront and get a code to open the locker later.
I've had good experience especially with a company called Stow Your Bags, which operates in more than a dozen cities in Europe, especially Spain and Italy. In Madrid, for instance, the lockers were less than 200 metres from Atocha Station, and charged just under €10 for four hours for a locker that could hold 4 carry-on size suitcases. There are two smaller sizes that go for less.
If you've found similar services you'd recommend, please post a comment!