Imagine Steve, Bucky, and the other Avengers doing the Ice Bucket Challenge.
Sam gets nominated first, by an old army friend, and of course he can’t resist nominating Captain America. Just the ice jokes alone are enough to make it impossible to resist.
And Steve’s a little shit, so he ends his video- which goes viral- by turning to camera and saying in his innocent “Aw shucks, Ma’am” tv voice “I would like to nominate my best guy, Bucky Barnes, for the Ice Bucket Challenge; and also, Tony Stark, who I’m sure will also make a sizable donation to this worthy cause”.
And Bucky’s just like “You fucking punk, when I get my hands on you”, but he does it (obviously) because it is for a good cause, and it can’t be any worse than actually being frozen (Steve assures him on this point).
Tony’s just insulted that Steve would even doubt he’d be making a donation in the first place. In typical Tony Stark fashion, his challenge video involves buckets of iced champagne being poured over him by bikini clad women. Pepper’s not remotely within the realm of being impressed by this, until she gets to pour the last bucket over him herself and watches him struggle to contain a scream.
Everyone thinks Tony’s an idiot when he then nominates Natasha, Clint, and Bruce, but the video of Nat and Clint drinking vodka in a sunken bath filled with ice before shirtless Steve and Bucky come in to pour dustbins of iced water over their stoic heads arouses new internet memes (such as Natasha Romanoff jokes which seem to follow the same formula as Chuck Norris jokes) in addition to the already existing ones about Iced Bucky Challenges and such.
It turns out that Bruce does Hulk out, as feared. But the Hulk likes the iced bath, so they leave him with a rubber duck and monitor him until he reverts to normal so they can fish Bruce out. He duly nominates Thor, who has coincidentally just arrived.
Thor insists on his challenge involving dropping him into the frigid seas of the Arctic Circle in his underwear from the quinjet. Nat and Clint nearly trip over themselves scrambling to pilot him out there, because, despite their reputations for stoicism, they’re pranksters at heart.