Science Fair experiment for Rocket oven

Hello big perms,
I am really hoping someone can help! My 12 y/o son will be building a rocket oven and wants to double it for use in some cool science fair experiment. Got any ideas that would make for an intriguing measurable experiment?
Anything would help.
Thanks!

how about:

What about measuring the amount of fuel required to bake a pizza, and compare to the amount of fuel needed to bake a pizza with a cob oven (if you can find a cob oven - where I live in Portland Oregon that wouldn't be hard).