How about removing and installing
fuel injectors, checking
compression, adjusting valve clearances on the
diesel? Checking and replacing
cooling system's thermostat? Checking and replacing an
alternator? Checking,
cleaning and replacing a starter
motor? These are all routine tasks that you can easily do yourself. You could even go as far as taking the cylinder heads apart, checking push rods and replacing/regrinding valves - none of these are rocket science, either - but that would probably be more than one day can cover.
Also, you can ask him to show you how your killswitch works (what exactly does it do, and what to do when it fails). Other possible topics are "how my
gearbox and throttle handles operate", "what to do if throttle or
gear cable breaks", "how to check and grease the joint between engine and shaft", "how to check stuffing box", "what to do if the stuffing box leaks", "how to check hoses and seacocks".
And then, there's also the
electrical system, which is simple in theory, but can be one hell of a mystery in practice