There are many plastic container and jar sizes you can find in the supermarket. I know I have a peanut butter jar and a tartar sauce jar in my cabinet that would fit nicely nested in a can and act as a cup and bowl if I cut them down. I'm sure I could find 3 nesting containers of usable food in the store or in my cabinets. You could make it an activity with your son to find and make them. The smallest water filter (and the cheapest!) is the Sawyer mini-squeeze. You could fit 3-4 nested cut down jars, a cat can stove (or canister stove) and a mini-squeeze with it's rolled up squeeze bag and a few utensils in a coffee can w/lid (small plate, cutting surface). When I was a teen and wanted to backpack and had no money or advice, I pulled stuff out of the trash for a cookset. Got me started. You can glue thin packing foam to the cups to insulate them. Glue the foam to mark the 8 oz. line. Sorry if I got carried away with suggestions, it reminds me of being a kid and gathering things together for a trip into the woods. Made me very happy.....