You have all the ideas listed. At some point you realize as an owner you must suck it up and pay people when it is slow. Or you ask first for volunteers to take off. Then you send them home and end up with poor moral. You should not have them doing things not related to their job. Having them wash owners cars, hang Christmas lights at owners home, furniture movers, appliance repair, painting your house is asking for trouble should someone get injured.
Let them get involved in ordering truck stock. Labor proposals on small jobs might be a few additional small task they can do.