I know this is old, but thought I would respond anyway for future lunches.
It really isn't that hard to pack disposable. I think that you are stressing over it when the main thing is just that you can't send a thermos type bag with a frozen thing to keep it cool.
For disposable fieldtrips....The main thing is to make sure that whatever you pack doesn't get all gross and warm while waiting to be eaten.
Here is what I do:
Double bag your lunch. Large Brown paper bag for lunch inside a gallon size ziplock bag. This will help the wear and tear that could happen to those paper bags, and keep all contents inside. Permanent Marker on the plastic bag your child's name.
3/4 freeze a juice box and place it also in a sandwich or quart size ziplock because it will sweat as it melts and will get everything in the paper bag wet. This will cut down on this issue. This will act as your "frozen" item that should keep the other items cool.
You can pretty much pack anything you normally would considering you will have the cold drink to keep it cool.
sandwich of any kind....turkey, cheese, creamcheese and jelly, or any lunchmeat favorite. (Hormel has a nitrate free hormone free lunchmeat now that is REALLY good!!) I ALWAYS put a thin layer of at LEAST lettuce on my children's sandwiches. I can't stand to see them eat a plain sandwich with NO vegetable. \
So you already have the basic sandwich and drink taken care of......now some snack ideas that my kids aways enjoy.
Danimals (drinkable yogurts)- don't have high fructose corn syrup or any of the other crap other yogurts labeled for "kids" have.
fruit cups with a disposable spoon
Applesauce with a disposable spoon
peanut/nut free granola bar (Quaker has a natural version now which is FANTASTIC, my kids like it better than the original that contains high fructose corn syrup, etc.)
Apple or Cut up Apple
Clementine (SUPER easy to peel mini oranges, with no seeds)
Box of Raisins
Mini bag of cookies or popcorn or snack chips
Pudding cups/Jello cups
individually packed carrots with dip found in the produce area at stores
The one thing to remember, is that especially at 5, no matter what their appetite is, they tend to eat less at fieldtrips. Also a little rule of thumb is that I know fieldtrips are special occasions, so I don't hesitate to give my kids something out of the oridinary, just for that day like a piece of candy or a mini can of pop instead of a juice box. It's only one day and it goes along with the "special" day.
Being disposable doesn't mean you have to send your kid with a lunchable and a bag of chips with a "cooler" type drink. Just pack a 100% juice box (or there are also prepackaged milk by nestle or organic valley(?) that is juice box like and doesn't have to be refrigerated.) and make your own snacks or put in your own ziplock bags. Before you know it, you will become a fieldtrip wiz. I've been on so many and have seen all the lunches that have failed the test of the fieldtrip. I think my double bagging and way I do things has been by far one of the better and well preserved fieldtrip lunches.
OHH...and pack wet ones to wash hands before and after they eat. Not all fieldtrips allow for bathroom time to wash up before eating.
Hope it helps in the future