I always liked Warthog battles, even without indestructible vehicles, they're still fun. Coagulation, and only warthogs on the field. But it's better on Halo 1.
I also like Zombies.
You have Two teams, everyone against the one Zombie (Generally the green team). The weapons are shotguns and swords, no weapons on the field, 5-10 minute time limit (Depending on size and ability)
Whenever someone is killed by the zombie, they switch sides to the Zombie's side, and start attacking the Humans. The Zombie's goal is to convert all humans, and the Human's goal is to not be converted. A human killed by a human does not convert to zombie, and a killed Zombie remains a zombie.
There's also Cops and Robbers
Two teams again, one small group of cops (two-four people, depending on group size), and the robbers (Everyone else). Weapon set of Plasma pistol and shotgun. One life. No radar. 5 minute Time limit.
The cops wield shotguns, while the Robbers wield only plasma pistols (but aren't allowed to shoot). The objective of the cops is to heard all the robbers into one area. The robber's objective is to kill the cops. When the cop spots a robber, they yell "Freeze!", the robber is expected to stop immediately, and follow the cop's orders. The cop then directs the robber to the pre arranged area, and tries to keep them contained there. The robber may attempt to flee or retaliate, but the cop is then free to shoot at them. While in prison the robbers are usually being watched by the cops. They can either sit there and wait for teemmates to free them, or can try to escape (Either while the cops are away, or attempt to Kill the cops with their plasma pistols.
If all the robbers are caught and/or killed, the cops win, when all the cops are dead, or if there are free robbers after time runs out, the robbers win.