Does anyone know what level that the UFO's start appearing on so that you can collect them for the

Yes, there's no point using UFOs for the new chocolate box, even if you have them, as the rewards are much less valuable than the UFOs. Just let the time run out.

Some tasks, such as fish, use the boosters generated within the game (from fishing floats, for example). But others don't, such as UFOs (and check marks, I think).