1. The Dancing Monkey - Captain America: The First Avenger
Steve Rogers has been on one hell of a journey. From fighting in World War II to assisting the Avengers in the modern-day, he's changed more than any other character in the MCU.
But no matter what, he cannot let go of his past life. From his undying commitment to Bucky to his love for Peggy Carter, modern-day Steve is haunted by his World War II days, and this is evidenced by a small detail in Captain America: Civil War that further ties into Steve's past and links back to his time fighting for the U.S. army.
Let's rewind to Captain America: The First Avenger. After becoming a super-soldier, Steve is forced to don a colourful outfit and travel the nation performing shows, an attempt to raise American morale and convince more people to join the war effort.
After he's booed off the stage at one of these shows, he sits on some steps and draws a picture of a monkey wearing a Captain America suit. This is what he feels like when he's parading about on-stage; like an object being wielded by someone else, a monkey being forced to entertain.
And, quite brilliantly, this monkey makes a return in Civil War, sitting on Steve's desk in a quick shot near the start of the film.
More than just a throwaway easter egg, this drawing is significant here because, again, Steve will soon feel like an object being forcefully wielded by someone else when he's asked to sign the Sokovia Accords.
It's small details like this that really make the MCU special, and this might be the best one in the franchise.