Cloaked fun after the underpass
Expanded March 8th 2005
When you get to the third group of Marines, there's active camouflage just before the guarded underpass. You can use it to do some novel things; and because of the checkpoint that occurs at the underpass, you can potentially keep reverting to have fun again and again. Here are some ideas.
(1) Far end
The invisibility lasts long enough for you to get right to the far end of the twin bridges area, take cover behind some rocks, and start up with your sniper rifle and rocket launcher. That's tough on Legendary as the enemy approach you fast and aggressively (actually it's perfectly possible to dispatch a few of the nasties on your way in, with plasma grenades); but it's a whole lot more manageable on Heroic. On a good day, some of your Marines might even struggle through the enemy ranks to join you.
(2) Hidden Elites
You can go down into the long 'trough' to where two Elites lurk in mysterious passages. Give them a surprise bashing while still cloaked, then wait for more enemies to arrive. Often a Ghost will come along and hover at the passage entrance. Introduce him to the effects of a rocket launcher.
(3) Destruction from the bridge
You can get to the door past the two Shades, slip past the Elites (the invisibility will be wearing off about now), continue on and up the lift, and eventually use a sniper rifle and rocket launcher to take out the enemies way down below from the edge of the bridge. I do love sending in a rocket from miles off, then watching with the sniper rifle.
(4) Evil fun
Here's an idea that doesn't involve going far at all. For this one you'll probably want to've killed all the Marines so they don't interfere. Go through the underpass and have some evil fun with the enemies that are waiting. On Legendary there are (potentially) a few Grunts and Jackals, plus two cloaked Elites. Take the opportunity to go up close to a Jackal for a good look at Bungie's magnificent work. When you've had enough of that, you could tag him with a plasma grenade and watch his amusing reaction. Or brush up against him, causing him to raise the alert without really knowing what's what. Or simply wait until your invisibility wears off, and watch him get an almighty shock as he suddenly realizes he's got a 7ft cyborg standing next to him. Or go pester those Grunts and Elites. Lots of possibilities here; it just depends how evil you want to be!