One of my favorites, Doofenshmirtz plans on building his own country, much to his daughter's dismay. Meanwhile, Phineas & Ferb make a rainbowinator (Where have we heard that before, -inator...haha.) to make a rainbow for Isabella. And Perry the Platypus is nowhere to be found as his neck cone prevents him from entering the lair.
I like how the plots were reversed, in a sense. Phineas and Ferb made a -inator device and Doofenshmirtz's scheme is foiled by them rather than Perry.
Speaking of Perry, well...Monogram actually, I like how he sang Gitchee Gitchee Goo near the end. Just a neat little thing.