He can see the Porsche Museum the same day as the tour. Be sure to book the tour in advance. The Mercedes factory tour is also neat to see and The Mercedes Museum is an absolute must (I think it's better than the Porsche Museum).
Outside of that, it will be summer time, which means all of the biergartens will be open. If he has an opportunity, the Rathaus (city hall) downtown has a perpetually moving elevator that is a hoot to ride.
There is a hill commonly called "rubble hill" that is interesting to visit. It's real name is the Birkenkopf and it is built from rubble that the Allied forces deposited there after the war. It's easiest to get to by car, although I believe there is a bus stop nearby. It is quite interesting to see and offers panoramic views of the city and region.
Speaking of panoramic views, you can go up the Fernsehturm (TV antenna), which is the highest structure on the highest hill in town. It's cool but freaks me out because I swear you can feel it move. It has an observation deck and a cafe up high.
How long is he here and what will he have for transportation. He can do more or less depending on these two factors. If he has time, there is quite a bit to do outside of Stuttgart.
I've lived here for little more than a year, so we have spent quite a bit of time exploring our surroundings.
EDIT:
I just Googled AIAD... We are here for related industries
PM me if you'd like more information or have details about where he is going.