A good Easter egg should work even if the viewer doesn't get it. Meaning, the scene should make sense in its own right, and the characters should not be acting out-of-character or only bringing something up in a THIS WILL BE IMPORTANT LATER way. The scene has to fit where it occurs, even if it will make more sense later or with additional background knowledge. Legolas' "Crebain, from Dunland!" line is acceptable, because even if viewers don't know what crebain are or where Dunland is, it's clear he means, "Look, the birds!" and the viewers have enough clues to figure out that the birds are Saruman's spies.
Overly-pointed prequel-y Easter Eggs are more cringe-worthy. Like Thranduil telling Legolas to go find young Aragorn:
That sort of thing only works well if you're going for ridiculous slapstick humor, or in a situation where breaking the 4th wall is acceptable/expected.
(For an example, the stage production
'A Very Potter Musical' actually defines foreshadowing while doing it.)
Example of a TV show Easter Egg:
This scene makes sense in the context of this show alone, so if you've never seen the show it references, you wouldn't be lost or confused - you would just miss the underlying joke...being that Jeffrey Dean Morgan is the actor who played both John Winchester on
Supernatural and Negan on
The Walking Dead. That barbed-wire coated bat is Negan's weapon, and Dean Winchester says "Dad would love this thing." The camera draws your attention to it, so you know you're supposed to notice it, but it's not going to be important (or reappear) again on this show, so it's just a little
Easter Egg.
We have had Bilbo and Estel meet already, so if Gandalf is assisting in the protect-the-Shire-from-wolves effort, then we could easily have a reason for Bilbo and Estel to meet again. And I do think we should reintroduce Bilbo, as we are going to need a Shire frame at some point, in order to stop doing all of the Aragorn frames indefinitely.
So, Bilbo having a magic ring that allows him to vanish is not something that we can really have Estel figure out right now, because that would be too much to explain and is irrelevant to his storyline. But, Bilbo *does* have a magic ring, and he's in the most dangerous situation he's been in since returning home, so of course he'd have it on him and would be able to use it. Having Bilbo just 'pop up' suddenly in a scene, and then pass it off as 'hobbits are very quiet on their feet' or 'I was a burglar, you know' would be enough to have the reference there for those who know the bigger story, but wouldn't interrupt the story being told here.