What to see depends somewhat on where you intend to go and if you are driving or using public transport, but here are some suggestions from personnel experience.
Of course the Porsche factory tour and Porsche museum are a must see.
Also, the Audi folks were road testing Lamborghini’s on the Autobahn A6, northeast of Stuttgart. I really enjoyed the Deutsches museum in Munich. This is a very large museum with tons of unique exhibits, it was outstanding.
Deutsches Museum: Deutsches Museum
Touring southern Germany along the Alpenstrasse is great fun and you can also venture into Switzerland and Austria if the urge strikes. Driving along the Rhine is nice. There are lots of castles along the Rhine and in southern Germany. I'm saving the Black Forrest for my next trip.
If you are going north, Berlin is a great city to visit. Lots of history and a contrasting mix of old and new architecture. Leipzig is nearby but I never made it there.
I recommend a good reference book like "Frommers Germany" (no affiliation) and a good detailed road map or GPS.
Enjoy your trip.
