AN: So I had this idea a while back, and so It's basically how Cas would react to random things that the boys do. Feel free to leave suggestions in the comments.

Cas looked at the look of utter joy on Dean's face. Sam had begrudgingly allowed them to stop and spend the day at an amusement park they had passed on their way back to Bobby's. He had left them not long ago to do his own thing, but Cas had stayed with Dean. He would be completely lost otherwise.
"Dean, why are we here? This seems counter-productive." Dean laughed.
"To have fun. Don't have any fun in heaven?" He scanned the area, looking for what he would try first.
"I'm made to kill you repeatedly for fun. I'm generally opposed to the idea."
"Yeah, well, try not to kill me, okay? Oh, yes. Come on, we're going on the rollercoaster." He grabbed Cas' sleeve and pulled him towards the ticket booth. They paid and took their seat, Dean practically jumping up and down with excitement. Cas kept a straight face, only breaking it to look suitably uncomfortable when a seventeen or so year old bent to adjust his seat belt.
"Dean," He muttered, "This doesn't look safe. Are you sure this is a good idea?"
"Cas, seriously, it'll be fine. I went on one when I was eleven. Rode it twenty times then threw up. It was great." Cas nodded and gripped the bar. Dean patted his hand gently.
"You'll be fine." He held he own bar as the roller coaster started, moving slowly. As they headed up the incline Cas relaxed and leaned into Dean slightly.
"I suppose... This isn't too bad." Dean smiled.
"Hold on…" They reached the top and lurched forward, their car shaking as it sped downwards. Cas' face went white and he held on to the bar. He glanced at Dean, who seemed to be enjoying himself, and let out a small whine.
"Dean, I don't like this. I want to get off."
"There are too many people around. Just hold on, It's almost over." He put a hand on Cas' and patted it softly, tightening his grip when they turned again. He left it there for the rest of the ride, withdrawing quickly when the ride slowed. He jumped out when they were still and wandered down. Cas staggered after him, an arm wrapped around his waist. He steadied himself against the taller man and they walked slowly away from the rollercoaster.
"Come on. I'll get you a hot dog." Cas looked up at a ferris wheel as the colour returned to his face.
"Why aren't they moving? That doesn't make sense." Dean laughed and handed him his hot dog.
"Come on. Let me show you."

So, ahem, let me know what you think. Reviews are brilliant, and if you want a Cas reacts chapter, let me know. Thanks for reading!