Suggestion, and you can take it or leave it, but in my humble opinion a true Easter Egg has nothing to do with the plot itself. People passing around sandwiches while watching a concert in How I Met Your Mother is a "callback," not an Easter Egg. A callback is when an idea is revisited without explanation, just to see if you're paying attention. (In HIMYM, "eating a sandwich" is code for smoking marijuana).
William Shatner guest starring as the Big Giant Head in 3rd Rock from the Sun, complaining that he saw someone on the wing of his airplane, and John Lithgow saying "I saw it too!" is an Easter Egg. The two actors played the same character in different iterations of Nightmare at 20,000 Feet on The Twilight Zone.
The scripture quoted on Nick Fury's headstone in Captain America: The Winter Soldier is the same scripture Samual L. Jackson's character repeatedly quotes in Pulp Fiction.
Etc