@SorryforSmelling Make a door in 3D - up to you to decide how the interaction with it works - do you press a button to open it, does it open automatically, ...?

Then add a mechanic for the door being locked or unlocked, and a key that the player must pick up in order to unlock the door.

Why this challenge? Because it's a fairly small and self-contained thing, that can nonetheless involve a lot of different engine features. Try to make it really nice to interact with!