Mabel and Dipper Pines switching places with Phineas and Ferb.

Just

Phineas and Ferb trying to harness the weirdness for their inventions meanwhile being completely unaffected by it because they've seen weirder, honestly.

Mabel coming up with the weirdest ideas and Dipper constructing them, meanwhile Pacifica attempts getting them in trouble half because pride and half because she's scared for their lives.

Phineas: *Finds the book in a fucking tree* Welp, I know what we're gonna do all summer.
--

Mabel: Okay, here me out; The world's biggest rollar coaster, held together by PEANUT BUTTER!
--

Bill: *exists*

Phineas: Wait, how do you float? What deminsion are you from? Why are you a triangle? Are you from Flatland? What are you made of? What exactly are your motives? How do you speak without a mouth? *continued questions of Bill's existence*
--
Dipper: *carries around a journal full of Mabel's crazy ideas*