What’s the Word for Hidden Treats?

Exhibitions are like other forms:
All follow certain given norms.
Seldom do we hide special secret layers,
Though visitors like to be game players.
Explaining up front there’s a hidden trick
Really can make your messages stick.
Everyone remembers the experience better.
Give a clue like “read down, each first letter” —
Guaranteeing your visitors will look with special care
Searching for the answer they now know is there.

Warmly,
Jonathan