Challenge your group to design or bring in a packed lunch. That will not contain anything that has to be thrown away in your general waste bin. Extra points if their lunch does not contain anything that needs to be recycled either.
For example using a reusable sandwich box and drinks container.
To make it fair you can make a rule that each lunch needs to include a main course, a snack, fruit and a drink.
Follow the tips to try to reduce the amount of rubbish you throw away by thinking about packaging that can be reused and recycled (including composting) when putting together a packed lunch.