1. Cap lifting Mjolnir. It can either be presented comically as he casually hands it to Thor, picks it up to look at it, moves it out of the way, etc, or in a somewhat more serious/awesome light during the heat of the final battle. Say perhaps Thor is downed and Cap picks the hammer up to deliver a blow to an oncoming enemy before handing it back to the God of Thunder. Not too much emphasis is placed on it, presenting it as subtly awesome rather than HEY EVERYBODY, LOOK WHAT CAP CAN DO. LOOK LOOK LOOK.
Double points if a) He's unaware of the significance of being able to lift it and b) It's already been shown that Tony can't lift it, and tries (unsuccessfully) to do so again after seeing Cap do it effortlessly.
2. Steve being bemused at fighting alongside Howard Stark's son.
3. Tony, Cap, and Thor drinking together. Tony's alcoholism, Steve's inability to get drunk and Thor being...well, Thor give it an interesting edge.
Honorable mention:
-Someone yelling "Avengers, Assemble!" I'm pretty sure this is a given, though. They can't NOT do it.
-Some kind of fastball special-esque move that requires either all 6 members of the team or maybe just Tony/Cap/Thor contributing one movement each to take down some kind of large (non-Hulk) beast or robot during the climax.
-On that same note, a balanced final fight. Never leave it up to just one hero to take Loki down.