My dad is 60 next year (sshhhh don't tell him I said that!) and I thought some father / son petrol head time would make a good gift. My original thought was a Europe drive and Italian museum / factory tour, but I think that was a bit too crazy as he has a replacement hip and can't really sit around in one place for a long time without it becoming painful, so I'm looking for other interesting options that don't involve me scaring him half to death over 2,000 miles in my passenger seat, probably something more like a flight to somewhere and a rent-a-generic-hatch to get us from hotel to hotel and museum to museum.
So...
He is fascinated by the pre-war German Auto Union / Silver Arrows battles and cars, and I've heard from a not that really interested in cars colleague that Mercedes Museum is quite something.
I did some googling, and discovered that BMW, Porsche, Merc and Audi all have museums next to / near their factories, and are all within about 2 hours of each other.
So, has anyone done the German 4 and if so, are they worth it? Did you factory tour as well, or just museum and are they big enough to warrant a day each? 4 factory tours and museums is maybe a bit overkill, and I was thinking BMW, Merc and Audi plants are probably quite similar, so any recommendations on which ones to tour if possible (I suspect Porsche might be different enough to warrant 2 factories maybe...)
Basically, any and all advice welcome

Chris