I'd like to have a group of debris fall from a ceiling onto the floor, but not have them drop all at the same time. Is there a way to set a keyframe that dictates when the object enters the simulation? For example, object 1 starts falling on frame 0, object 2 on frame 5, object 3 on frame 24, etc.
Yes, you can keyframe the Animated checkbox by hovering over it and pressing I:
When Animated is enabled, the rigid body will be controlled with the animation system. When it is disabled, the rigid body will be controlled by the rigid body simulator.