Suppose you could take an egg and a coin (which can be borrowed) and have both of them marked by a spectator. You then cause the coin to magically penetrate inside the egg.
A small hole is chipped in the egg, big enough to pour out some of the egg goo, but small enough that a coin, which will be found inside the egg, cannot escape. Spectator can look inside the hole and verify they see the coin and their mark. Spectator can reach through the hole and grab the coin with tongs or tweezers and pull it out through the shell themselves and can immediately verify that it is the same coin they marked earlier.
No tricky sleights. Yes, there is a gimmick which you will have to create, but this takes less than ten minutes once you have the knack, and many can be made at once and saved.
You receive a nice 45 page (complete with over 50 photos) booklet giving you two versions of this effect, one totally impromptu, and another which thoroughly explains the gimmick.