I just returned from a month long trip and a lot of the information I had been told was not the most accurate. A lot has been answered but to quickly recap some of your questions.
Passport only needed for Japan
I paid 800 roundtrip out of LAX with United. 12-14 hour flight
Narita is extremely quick for customs, I flew in on a Friday at 5pm.
$500+credit card should be plenty if its 1-2 weeks. Japan may not be super credit friendly but Tokyo is. You mainly need cash for food and if you go to the nice places they take CC. I don't think I found a shop to buy stuff at that didn't take CC. Only one street vendor I bought some artwork at was cash only. Akihabara-Credit Friendly, Hotels-Credit, Nice restaurants-credit, Karaoke-credit. Don't know what Japan most people here have been to but credit card was everywhere. I brought $3,000usd, changed $2,000 to yen...400 was stolen, and I still left Japan with $1,300 blowing the remaining $300 in yen at the airport on trinkets. I also traveled from Tokyo all the way down to Kyushu region and back. I did a ton of things and CC was pretty much everywhere. SHORT ANSWER: Street vendors and small restaurants are Cash.
Where to stay....HOSTEL!! I had originally booked hotels cause I figured I had the money and hostels were for people on a budget. I stayed my first night in a Hostel cause I figured I would be arriving late and just sleeping(I also wanted to stay at each type of place) figured one night wouldnt kill me. It was a great place, super clean, modern, and the staff was super helpful.
I then stayed at my fancy hotel...staff was much more limited in their English. When I asked about places to see or do I was pointed to the canned tours. In comparison the hostel staff was younger, cuter
, and more in touch with the cool things to see and do. Plus at the hostel you meet tons of other cool travelers, many solo as well, giving you the option to hang out if you want to have company.
I canceled many of my hotel stays and opted for hostels not because they were $30 a night compared to my $300 but because they were the better experience. Japan hostels are not like US or Euro ones...clean and people are pretty respectful at them. Ninja Khaosan in Asakusa even has the "cabin" style...pretty much like a capsule, so you have decent privacy.
Thats my two cents...oh yea and eat at Ippudo for ramen (no one ever says it) I found that place to be the best for ramen about $10us