I know it's a bit late for this project, but if I might offer my two cents for future readers: I think it's always worth considering how best to spend time and effort. What is the maximum you can offer in the minimum amount of time? That's not at all to say offering sub-par or cheap rewards. But rather, what can you offer that's meaningful, but doesn't require you to take too much time away from making the whole project as good as it can be?
Weapon skins seems like a solid idea, as it can be fun and interesting for players, without requiring days or weeks of work. Of course, if your 3D artists are already finished and it's just down to polishing the game, go crazy.