"I'm Just Your Problem" is one of Marceline's best. And for those who feel that the song is too self pitying without letting Bubblegum say her piece, "You’re Your Own Problem" is an AU fan song where Gumball either gives a "reason you suck" speech or has a "reason you suck" debate with Marshall Lee, telling him that only he can fix his problems and culminating in a declaration of devotion and love.
"Time Adventure". "Time is an illusion that helps things make sense. So we're always living in the present tense. It seems unforgiving when a good thing ends. But you and I will always be back then." A beautiful heartbreaking lullaby BMO sings to both Jake and the audience, reminding all the longtime fans that though the story may end, it will always live on in our memories.
"Waving to You", the show's very last song, debuting in the credits to the Grand Finale instead of the usual tune "The Island Song" (since it was already played in its entirety during the ending montage); its emotional, bittersweet and melancholy tone combined with the title is pretty much a metaphor of the show and it's characters waving goodbye to us. It's especially an absolute Tear Jerker for people who grew up with this cartoon.