Are there any grains or grainlike produce (not kitnios) that cannot become chometz?
Echoing NoachMiFrankfurt's answer of Quinoa, the grain-like food isn't kitnios and doesn't become chametz. In an article by the Star-K titled "Quinoa: The Grain That's Not" Rabbi Rosen writes,
Quinoa was determined to be Kosher L’Pesach. It is not related to the chameishes minei dagan-five types of grain products, nor to millet or rice. Quinoa is a member of the “goose foot” family, which includes sugar beets and beet root. The Star-K tested quinoa to see if it would rise. The result was as Chazal termed, sirchon; the quinoa decayed – it did not rise.