Fun
Here's an extensive collection of ideas for getting additional fun out of Halo's campaign mode, covering serious combat, cyborg mischief, odd things to see, and more. Most of this comes from my own explorations, but some of it stems from things I've read about or that people suggested to me, which I try to make clear. Levels 2 to 5 offer by far the most extra fun. In the level-specific sections, items in a given class (highlights, general, or minor) are listed by how far through the level they relate to, as best as possible.
Level by level
Fun for multiple levels
Playing through levels in different ways
Quick links to the glorious megabattles: rockslide megabattle, dustbowl megabattle, cavern megabattle, tunnel megabattle, tower zone megabattle, spiral path megabattle, twin bridges megabattle.