...Or an anthropomorphic duck.

Yeah. My trick only works when you can use air pressure and control the openings of a volume. Drop ceilings are another matter entirely.

Another trick is to use an air compressor. As long as the path has only one way for the air to escape, the string will completely follow the air path to that exit. So you may need to tape up any spots that you don't want the string to follow.

It will make some crazy zig zaggy paths (if needed) to reach the end. But it works.