Is the goal to reduce total trash, including the stuff you threw away at home, or just to reduce the trash during the outing?
If I buy a box of cereal, transfer it to a reusable container, throw the box away at home, and take the reusable container on the outing, I've reduced the trash on the outing, but not the total trash.
What do you do if you're already doing a pretty good job on the first outing, and there's really not a lot to reduce? Is it enough to discuss the trash you generated and that there wasn't a way to significantly improve on the second outing?