Okay, these is a complete overhaul of my last post.
Never, Ever, EVER say you like Anime. Anime is considered a childish activity and is frowned upon by people older the age of 12 unless they are an Otaku.
Speak the language. Even the smallest bit of speaking the language and just kindergarten level speaking is better than not being able to, and if you know some of the language people tend to have more respect for you.
When buying electronics try buying used from trust-able shops. If you play games the average xbox game can be about 7500 yen, that is about $88. Try buying used games, there were shops in Akihabara that sold used games and even retro games.
Tour the sites. Try visiting the city every once in a while, its always good to get out and visit the sites. I would definitely visit the Hiroshima Peace Memorial Museum, my class visited there and it was an eye opening experience for us all.
If you find a lost item on the ground leave it there. People tend to leave items in nooks close to the place they were dropped or left so that the person who lost it can find it.
Be respectful, always say "Sumimasen"(Pardon/Excuse me) whenever you bump into someone or need help. Be kind and respectful, a lot of Japanese people are xenophobic in some manner.
And the most important: Know and obey the laws.
learned all this from my japanese teacher who is a native when we stayed in Japan for 2 weeks as an activity for our school class.
[Edited on 01.07.2013 1:59 PM PST]