Fun
Here's an extensive collection of articles on getting extra fun out of Halo's campaign mode, covering serious combat, tricks, exploration, novel activities and more. Most of this comes from my own explorations in the game, 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.
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.