Easy, peasy. Schedule your rides and other desired activities and downtime in advance and then tell the offerers of volunteer opportunities what your time frame would be to complete the job. If they can't take a slow pace they can always take up a collection and pay someone to do it on their schedule. I am the PC fixit guy for my neighborhood "help the old folks age in place" nonprofit. I am always available to help but often with the caveat that "I will not be able to get over there until Monday morning," or whatever. Once in a while other volunteers step in if there is a need and there is always the Geek Squad.