A few car related suggestions
The "fancy nissan showroom" you're talking about is the Nissan Gallery underneath their HQ, it's about 5 minutes walk from the smaller nissan gallery on harumi dori (they have a map). It's address is Chuo-ku Ginza 6-17-1. I usually pop in for coffee on the way back from breakfast at Tsukiji to see what they have on
I'm assuming you've done tokyo megaweb and the historic garage/museum in odaiba, if you haven't you should. Welcome honda is next to aoyama ichichome station as has a few things there, cars, asimo demonstrations etc.
If you feel like stretching to Nagoya the Toyota museum there is fantastic and the Toyota commemorative museum of industry and technology is one of the best "applied" museums I've ever been to
There are a number of car museums around Mt Fuji covering standard museums to racing museums to museums dedicated to italian exotics. Best done in a hire car as they're in out of the way spots but worth the trip