One of the largest man-made threats to our future right now is plastic waste.  At least 14 million tons of plastic end up in the ocean every year.  As well as having a negative effect on marine life, this is harmful for people too; the majority of tap water sources were found to have microplastics in them.

Many, many people drink milk.  Go to any grocery store, and you'll see an entire shelf of plastic milk bottles.  Once the milk is finished, the plastic bottle goes in the trash, or worse, in the ocean.

I would like to propose that grocery stores add milk dispenser machines to their stores.  They would work similarly to the soda dispensers you see in fast food restaurants: a shopper brings an empty reusable bottle to the grocery store, puts it under the dispenser, and pushes a button to make milk come out of the machine.  The customer then pays an amount based on how much milk came out of the machine.  The next time the customer runs out of milk, they simply wash the bottle and bring it back to fill again, with nothing thrown in the garbage.

Such an addition would attract environmentally-minded customers.  It would also reduce the amount of plastic going into our environment, with very little effort on the consumer's part.  It is already a thing in New Zealand.  Even if not everybody is on-board at first and some are still buying plastic milk bottles, every bit of plastic kept out of the environment is important right now.