The easter egg is a different ending when you finish the last level on the hardest level - which is actually pretty easy because the last level is really just running for it.
I like to blow the warthogs away using plasma grenades - in fact the plasma grenades are great fun. For example, get a 4 player game started with unlimited grenades, then get everyone to throw grenades at the same spot - then watch the fireworks
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I like to play ghost basketball too, make a game type with all the vehicles, and start on the Blood Gulch level. Now park a Scorpion at the side of a base, then use that as a ramp for the ghosts to get onto the roof. I like to launch the ghosts through the hole in the roof - especially when someones in there tooling-up.
Van-B
The nature of Monkey was irrepressible!.