Fruity Rainbow Flakes are a cereal Phineas & Ferb eat for breakfast. They are seen eating it in the beginning of the episode "Hail Doofania!". The cereal also powers the rainbow-inator, made by Phineas and Ferb.
(Phineas and Ferb eating Fruity Rainbow Flakes)
Isabella: What'cha eatin'?
Phineas: Fruity Rainbow Flakes, you want some?
Isabella: Oh no thanks, I already had breakfast.