Play as Flats (In BfBB)
While standing in front of Mrs. Puff in any level, input the code "X, B, Y, A, A, X." Now, talk to Mrs. Puff. After a different conversation, you play as Flats. To switch back to Spongebob, repeat the process. While using Flats, you don't have any unique abilities, but his punch will immediatly launch and kill any enemy, including bosses.
Play as Scooter (In BfBB)
While standing in front of Larry, input the code "B, A, Y, A, B, X." Now, alk to Larry. He'll explain that he saw Scooter around the area. Search the beach until you find him. To use him, talk to him. To change back, talk to Spongebob, who will be standing in Scooter's place. While using Scooter, you can surf on water and access hidden "Surf for Shiny Objects" courses.