A space station with one large central room, and several outlying areas. Nice views from the hallways... and the staircase in the central room adds to the flow dynamics. (Not quite as effectively as it could, since the switch to raise and lower it is on the bottom level-if it were on top, you could trap people in the hole.) Big enough, and spread out enough, that two players might get bored.
A nicely done outdoor canyon. Frenetic at first, because of the baddies waiting for you, but once they're gone, it's quiet again...There's a fast-moving river through the middle, with access to an underground cave, and plenty of nooks and crannies. A single manmade room overlooks the field, with teleporters to various high points. Probably needs three minimum to obtain decent carnage rates.