Introduce bragging rights! For example, a human could get to e.g. 250 score and brag: “There’s a pirate ship in my sock!”, and then throw a sock — it would then explode into a pile of mangled wood and metal, damaging both structures and players, obstructing passages (should be possible to clear of course), and, of course, releasing parrots who would then attack aliens and their structures. To use it again, they would have to get to 500 score, then 750 for a third use, and so on. You could also make the trigger phrases configurable, change them regularly, and leave clues at the forum/Wiki/website…
These would be much better cheats than the floor CHEATbite!