Every time I see posts of people leaving stuff, it's always big untouched multipacks of bottled water and I wish people would stop buying them, you don't need them. You can get free ice cold water at your resort soda fountain in the food court, you don't need to buy a Disney refillable mug for the water to work, there's a little button under the Minute Maid Lemonade that say 'water'.
Bring each person a refillable water bottle (and even a big empty soda bottle to fill and keep in the room fridge) and get the free water in the morning and evenings from there and you can even refill them in the parks at dedicated water bottle refill stations and self serve soda fountains (Cosmic Ray's, Connections, Sunshine Seasons, Backlot Express, Satu'li Canteen, Restaurantosaurus) or ask as CM to fill at any quick service counter.
This reduces plastic waste, saves money and then not having to worry about how to dispose of your unused crate of 30 plastic bottles of water.