"I have an idea," Charlie points out after several minutes of discussing how the hell you surprise an angel. You can't exactly expect him to shit his pants if they just hide behind the sofa and then yell SURPRISE, he simply wouldn't understand the concept of it all!
He's just a baby in a trenchcoat, who doesn't understand humans whatsoever. Guessing that angels don't celebrate birthdays as they do, he probably wouldn't even realise it's his birthday in the first place. If they threw him a surprise party he would only wonder why they did all these weird things. They need something more, something for Cas to remember. But hell, he's been through a civil war in heaven! How do you top that?
Besides, as Sammy pointed out, who would they invite that isn't the four of them? Cas' birthday is tomorrow and so far none of their ideas have seemed to work out even in his mind. Which is why Dean is so relieved, and at the same time taken aback by Charlie's inventiveness when she reveals her plan.
"Let's throw a cake in his face!" she yells and freezes into a pose with both her arms in the air and her body leaning towards the table, waiting for Sam and Dean to show some sort of reaction. Sam just sits there, jaw dropped because of the plan as Dean smirks and exclaims;
"That's awesome! I always wanted to throw a cake in his stupid angel face."
Sam frowns at him and Charlie, somehow Dean knew his little brother would back down. Sammy is too kind for these things, way too kind. The older brother gives him a "come on man" kind of look, even though he knows it will be useless.
"...I mean, we don't exactly have a better idea."
Either way, it's two against one, so Sam has to roll with the plan and be part of it whether he likes it or not. However, it doesn't really take long for him to warm up to the idea, and it's not he who should take the credit, it's Charlie. It's such an awesome plan, Dean thinks, and Cas will in fact shit his pants when the cake collides with his face, all thanks to the little genius.
***
The big day arrives and the three of them meet in the Men of Letter's main room as they had agreed on the day before. Dean is glad to notice Sammy's change of mood about the idea. In fact, he's never seen his brother this excited in his entire life. The Winchesters and Charlie stand by the front door, Charlie ready with the cake.
It's covered in a lot of cream, and even filled with cream. Cause why not make it the creamiest cake in the world, that way it's perfect for its purpose of attacking a person's face! Well, not just any person… if you can call Cas a person...
The door opens, the assbutt enters the room and gives them his ordinary confused face and it's the perfect face, considering what's going to happen. Dean starts to wonder why she hasn't thrown the damn cake yet, she better not chicken out, not now when he's—
"Surprise bitches!"
Just like that, the Winchester has cream all over his face, even in his mouth and nose, freaking everywhere!..
Dean wipes the cake out of his eyes to see Charlie and Sam folded in laughter and giving each other a high five. This was not the plan!
He turns over to Cas who just looks to be even more confused than he was before, glancing across the room and seeing a big banner that says "happy bday assbutt angel". Cas looks down at the floor, caught up deeply in his thoughts and frowns.
"I don't understand…"
Dean feels his rage stirring up inside of him, but then he sees Charlie and Sam laughing and the anger blows away. How can he ever stay mad at those two, especially Charlie. It was a perfect plan and a great surprise for them both.
This day will be awesome, even though his face is covered in cake.
Cas looks around the room once more, then at Dean, and if he's not mistaken he thinks he can see a little smile on his face. Maybe his birthday wasn't so bad after all — in fact, the guy in the trenchcoat can't remember the last time he felt this loved and truly happy. Even though it might now show on the outside, his heart is just overwhelmed by love.
And from that moment on, Cas knows that this is where he belongs.