I think the main issue here is that people are not really valuing their own belongings, nor caring about their environmental impact, and just leave things they don’t want in the hope that someone else will be grateful for them…and if not, will just bin them.
Buying a bottle of shampoo just for a holiday, not finishing it, and then leaving it on the off chance it will used by someone else is pretty wasteful when there’s a chance it will be binned. Presumably, you’ll need to buy another shampoo to replace that one? Why not just take a smaller decanted bottle with you, or take the half used bottle home to finish? Presumably you had more in the bottle in your luggage on the way out?
Leaving outgrown kids gear on the off chance the cleaner knows a child the correct size, but they might just bin it instead. Why not take it to a charity shop and donate it properly?
Leaving unwanted food that might not eaten. You bought the food, eat it, or take it home.
If you want to be properly generous to the staff, give them a monetary tip and stop leaving things that they may, or may not, want.
I get leaving a communal library of inflatables or sand toys next to a pool/beach. You’ve actually found a market where lots of different people who are off to enjoy the pool will likely want to borrow the items and they are unlikely to be binned by a busy person just trying to get on with their job of emptying and clearing.