Lock Before Key
Here's a recent addition to My Game Design Principles:
Present the player with the lock before the key to open it. A key without context is just trash. "What's this key for? I don't need it."
A lock can be intriguing. "What’s behind that door? What’s inside that chest?" Locks ask questions that only keys can answer.
Locks can take many forms: impassable rivers, high cliffs, invulnerable monsters. Keys also: rafts, jumping boots, flaming swords. Provide the need before the solution.
More here:
Make One Game at a Time
Game design principle: don't kid yourself into building multiple games simultaneously.
Singular Capability
Game design principle: each entity should do just one thing.
