Or maybe finding a fossilized skeleton of a Native American with a shotgun. A callback to Turok.
Or how about this, and this has a bit of puzzle element to it. Ill use the 3 raptors we conclusively have names for: Raptor, Acid Raptor, Electric Raptor, as an example.
In order to get this achievement, you will enter a room. There are the 3 raptor types. On one side, off somewhere in the room, out of initial site is a door with 3 color coded dots and in this order. Blue, Yellow, Red. Blue = Electric Raptor. Yellow = Acid Raptor. Red = Regular Raptor. You can damage the raptors and it wont hurt this unlock, but what you NEED to do is kill them in this order. If you dont, you will have the reset the stage, otherwise the door will be locked because the raptors in this room dont respawn, but if you kill them in this precise order, the door opens up. Unlocking your achievement, maybe an easter egg, maybe some unlockables, etc. Its all up to the devs. This would come off as nothing at first inspection, but upon further exploration, and revisits, and possibly browsing the achievements list, you would actually discover this area had more than meets the eye.