I am having trouble coming up with a noun that is almost like an Easter egg. Is there any word that could be something someone intentionally leaves for another person to find at a later time?
[A] word that could be something someone intentionally leaves for another person to find at a later time
treasure (as in buried)
discovery (at first not such good match; but ...)
galleon (as in treasure ship)
Also, check synonyms of all the suggestions you get.
Drawing a longer bow
rainbow (as in rainbow's end)
A gem is the only commonly known word for a hidden treasure. However, people in Europe and in the Mideteranian countries will recognize (truffle) as a good word for your app.
Consider the following: hidden message, cryptic code (or crypt), cipher, secret, puzzle, inside joke, Cicada 3301 (if you're up-to-date with Internet culture), and the Lost Ark (if you're an Indiana Jones fan).
EDIT: For your app name, here are some ideas by playing with the phrase "Easter egg":
- Easter Leggs!
- Easter Feaster!
- Easter Teaser