All good fun - if your wife is obliging. I'll reinforce the comments above with these nuggets:
1. Loved the Mercedes Factory tour at Boeblingen, not just for the cars but also for the case study of scale and logistics. You can see videos of the production line on Youtube but visiting it you realise how huge and immaculate it is. Watching the punters collect their new cars is a curious ambivalent pleasure. There are some strange new owners out there. Could easily spend a day there
2. Next to the MB factory is the Motorworld and the V8 hotel, a car themed hotel and enormous complex housing an arrange of car dealers, both modern and classic. Again could easily fill a day:
V8 Hotel-Motorworld Stuttgart
3. The Mercedes Museum is again worth a day, and is everything that you'd expect from MB
3. MB Classic also operate in Fellbach. An arcane choice. They specialise in renovating anything, of any age. Mind-blowing.
Mercedes-Benz Classic Center.
4. If you're an AMG owner, the AMG factory tour at Affalterbach is well worth the half day. Short drive outside Stuttgart and they treat you like a King. It's strictly engines and the most extraordinary bespoke builds, not "normal" AMG builds
AMG Factory Tour
5. Porsche Museum: obvious. Even more crazy history and money about. A day would be easy, but many will just split a day between the museum and the factory tour.
6. Porsche factory tour. Exceptional if you have the chance. Again a logistics tour de force. Very disciplined tour - wander off and they'll set the metaphorical dogs on you. "We must be disciplined in walking through the factory."
7. BMW in Munich - haven't been but intend to soon.
8. VW Autostadt, Wolfsburg - said to be the biggest tourist attraction in Germany - those crazy fun-loving Germans. Find that hard to believe, but that's a claim I've heard. Everything from VW bread, through VW sausages to even cars. Currently partially closed due to some health pandemic thing but will be open again later in the Summer.
Current News
9. Apparently there's a company called AUDI at Ingolstadt which does tours and has a museum. Can't say that I've ever heard of them. Sounds like some kind of VW spin off but apparently they have an interesting history.
10. And, of course there's the Nurburgring as well as a number of derestricted motorways to be visited. I wouldn't do Nurburgring, I'm happy to watch the annual crash compendium, but each to their own. But Germany's derestricted motorways will disappear within the foreseeable future, so maybe worth a visit.
11. Loved the Schlumpf museum which takes a full day. An extraordinary story of obsession with a unique set of cars. Stay in Mulhouse if you can. (Pronounced Mul- Ouse, as in the river in York): fascinating border city, that is half German half French, historic but very much off the beaten track.
12. If you're really techie there's a great Auto & Technik museum at Sinsheim. Arcane but all kind of interesting stuff including a Concorde that you can walk through. (If only to horrify yourself that it was still flying commercially twenty years ago)
13. And then there are the annual car shows: Geneva, an easy day trip, Frankfurt, Paris, Retromobile etc etc.
14. Can't talk about Italy, Maranello and Modena and all that - not my kind of motoring - but if you do go down there, note that you can hire a Ferrari for a 10, 20, 30, 60 or more drive. Expensive, naturally, but it appeals to some.
15. If you're in Italy in June, it's worth "dropping by" the Mille Miglia. It's quite a bun fight. 15 -18 June. It's the 40th re-run this year.
1000 Miglia 2022 – 15-18 June 2022 – 1000 Miglia
16. If anyone knows how to get into Bentley or Rolls Royce in the UK, short of ordering one's next Motor, I'd love to know how.