Small water bottles that fit in the lunch box
We pack a small water bottle inside each lunch box, so the kids have water with lunch and the bottle comes home with the box instead of getting left at school.
https://www.afterpickup.com/small-water-bottles-in-lunch-box/
We pack a small water bottle that fits inside each lunch box. It holds just enough water for lunch and sits easily alongside the food. The kids still have their full-size water bottles for the rest of the school day and for after recess, so the small one has a single job, which is making sure they have something to drink with lunch. We try to avoid juice and milk at lunchtime, so water is what goes in the box, and the bottle and the box leave the house together and come back together at the end of the day.
At school, lunchboxes and water bottles are trolleyed to the lunch room, and after lunch the kids are responsible for putting the lunch box and the water bottle back on the trolley. When the lunch bottle lives inside the box, it goes back on the trolley with the lunch, so it comes home instead of getting left behind.
Sending a full water bottle to lunch was heavy and cumbersome, and every so often the kids would forget it and only realize once they were home. The small bottles stay with the food, so the big bottle is free for the rest of the day and there is nothing separate to remember on the way back to the trolley.
All the kids love it, and they think it is weird that other parents do not do the same, because they see other kids forgetting bottles and dealing with them every day. It is a small change, but it improved their lunch time.