Is it possible to make a lid that is light enough for the air coming off the fan to raise it attached with hinges so it drops back down when not in use. This would keep debris from getting inside, yet still be strong enough to close and protect the unit.
I really would not block the AC vents, even with a light cover. Restricting the airflow will definitely impact the efficiency of the unit, and probably shorten its life as well. (Even if the cover is light enough to lift up when the fan is on, you will be reducing the airflow.)
If you have a problem with leaves and debris getting in there, maybe you can fit some wire mesh over the openings?