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