I have an affinity for advertising materials. I haven't spent any time going to design school and I don't think I've ever had ambitions to be in advertising, but I admire those who put effort into creating beautiful works of corporate art.
Located just south of the Ginza area, ADMT (The Museum of Advertising and Marketing, or just Ad Museum Tokyo) collects some of the best examples of advertising in Japan and the world, and provides a good history of advertising in Japan.
I spent about 45 minutes here, browsing through all the displays and watching some of the commercials. There are also interactive displays allowing you to look at additional material.
The museum is open Tuesday to Friday 11-6:30, and Saturday and Sunday 11-4:30. Last admission is 30 minutes before closing, and when Monday is a holiday surrounding days could be closed. Check the schedule on their website for a calendar. The closest station is Shimbashi Station, served by JR, and the Ginza, Oedo, and Asakusa subway lines. From there, head to the Caretta Shiodome mall (part of, or attached to, the Dentsu Building). The museum entrance is on B1F, and if you enter from the fountain (B2F) level (Gate A), you'll take the elevator up one level and see the entrance on the right. A fairly good interactive map with photos is here to get you from the station.