Phineas and Ferb cake

I have taken on the challenge of creating a Phineas and Ferb themed cake for my favorite little kid, Ethan's birthday party this Saturday coming up! He is turning 2 and he absolutely loves watching that show whenever I babysit him.  The cake I am going to try to make is two tiers covered in fondant and decorated with fondant as well.  The exact cake I want to try to replicate is below:

I have already ordered the surfing Phineas and Ferb characters as well as a Perry the Platypus character to add to the cake!  I'm super excited to create this cake, but also very nervous because this is my first real cake project that someone has requested of me.  I just want to make it look perfect for them!!! I hope I can make it look awesome!