Fun with invisibility
Down near the Silent Cartographer is an active camouflage item which you can use to get right up close to an Elite and some Grunts that are hanging around nearby, up the ramp from the overshield. Close enough to smell their breath, if it isn't filtered out by your helmet. The Elite is usually just standing still, so it's easy to approach. Some of the Grunts do a sort of jumping exercise to pass the time, which is amusing.
When you're good and ready, whack an enemy on the nose and enjoy the reaction. You've got time for several whacks like that, not to mention a bit of plasma grenade tagging. You can set all this up so it happens just after a checkpoint, giving you the opportunity of reverting to the checkpoint to start the fun all over again.