I lived in Japan from the age of 10 to 14 but visiting as an adult and a tourist was such an incredible experience. I felt lucky to have a loose grasp on the language and understand a few of the cultural norms and expectations, but I saw everything with fresh eyes, completely as a foreigner instead of an expat (can a child be an expat?).
I am by no means an expert, but I thought I'd offer some practical advice if you're ever thinking of visiting Japan.
Direct flights to Japan are expensive, which was always my reason for not wanting to visit. We got flights with a short stopover in Istanbul through Turkish Airlines for £400! I still can't get over it. Not only did we save £600 each, the flights and transfers went without a hitch, our luggage reached its destination, the entertainment was good and the food was seriously impressive - especially the meals on our flight back from Japan. I would seriously recommend checking them out if you're planning any long haul trips as they fly to loads of destinations around the world.
If you're entering Japan on a tourist visa and planning to travel a bit when you're there, then it's worth looking into a JR Pass, which offers unlimited train travel for a fixed price. The pass is pricey, but a one week pass costs the same as a return ticket from Tokyo to Kyoto so any airport transfers, day trips and certain subway lines within Tokyo during those 7 days would be free.
At first glance, the Tokyo subway system is a bit mental, but it's sort of logical once you get the hang of it, though signs aren't always in English and as there are a few privately run train providers, it's not always clear how to change between them. Download the Tokyo Subway Navigation app - it's free and it works offline, but it only covers lines run by the Metro system.
Technology-wise, the Japanese think of everything and portable wifi is the most genius invention. It's smaller than a phone, so fits easily into a handbag or pocket, and is invaluable for looking up maps or restaurant recommendations. We reserved and prepaid for one online, which we picked up at the airport post office, but there's also a place to rent them inside the Narita airport train station (and I'm sure there would be at Haneda as well). We rented the wifi for a week, while we were travelling around Kyoto, and then returned it when we got back to Tokyo as our Airbnb came with one, which seemed to be a standard at all the apartments I looked at.
WHERE TO STAY
We stayed in this apartment through Airbnb and loved that we could live like locals, do laundry, wake up late and save A LOT of money, but we also treated ourselves to two nights in a hotel at the end of our trip. Hotels are expensive in Tokyo but if it's your first time in Japan it might be nice to stay in a hotel when you arrive to get your bearings. Booking.com is great for getting the best deal; I like to book a few rooms in different hotels as you don't have to pay upfront and there's usually no cancellation fee (though make sure you check as it varies from hotel to hotel), so you can pick where you went to stay up until the last minute. We liked the Mitsui Garden Premier Hotel Ginza for its great views, big bathroom and central location.
WHAT TO EAT
Of course I'm not going to tell you what to eat! Eat everything! But I just thought I'd round up a few things to be aware of. Usually, Japanese restaurants only serve one type of thing, so it's worth discussing with your travel buddy what you really want to eat and what you're not that bothered about; there are certain things I love that Darren wasn't keen on, so I was happy to grab a quick meal on my own on a few occasions!
Japan can be a tricky place for vegetarians as even seemingly innocent dishes like miso soup are often made with dashi, a stock made from kelp and dried bonito (fish) flakes, that forms the basis of many Japanese meals. It's worth doing a bit of research before you go if you don't want to end up eating anything with fish in it.
SPEAKING THE LANGUAGE
I was surprised by how little English people spoke...or wanted to speak! My Japanese is good enough to order food or ask for directions (though not always to understand the answers!) and I have a feeling it confused people when I would ask if they could speak English...in perfect Japanese. But that's as far as my Japanese goes. Japanese people often study English at high school, so I have a feeling that people are wary of saying they do speak English, and then not understand what you're talking about, so it's easier to say no. Saying that, Japanese people are unfailingly polite and are so kind and helpful; if they really don't understand English, they will still try really hard to answer your question. And lots of people are happy to practice...we actually had a few people come over to us and randomly start up conversations on seemingly random things.
The one instance where the language barrier can be a problem is when you're looking for a restaurant. If you don't know what the name of a restaurant looks like in Japanese, it could cause a bit of a problem, so try and look for a picture of the restaurant's sign if it's somewhere you're really set on going. We found it difficult to pick as place to eat when we hadn't planned where to go as signs and menus are usually in Japanese and a lot of the best 'hidden' restaurants have fabric over the doors so you can't see in and get a feel for what you might be stepping in to!
Japanese toilets deserve a blog post in their own right! They have electric toilets called Washlets which do everything from warm the seat and offer bidet functions to play music or flush sounds to disguise any embarrassing noises. You might find traditional squat toilets in some less advanced public bathrooms, like in parks, but for the most part public bathrooms have Western toilets and are almost always spotless - even in the train stations!
Japanese people are unfailingly polite - they see you out of their restaurants with a wave, sometimes standing outside until you eventually turn a corner, they hand everything to you with both hands as a sign of respect and they love a good queue as much as us Brits do.
Apart from the occasional sharp elbow when you're getting off the train, Japanese people are very aware of their actions and there are a few things they don't really do. Firstly, they don't eat on the train - except in the case of regional inter-city trains, where it's a big thing to get out your bento (packed lunch) and a few drinks and make a real occasion of it. Although I've seen people have a sneaky snack or a swig of a bottle of water, for the most part they really don't eat on the subway and certainly not a proper meal like a burger or a sandwich. Secondly, they only smoke in designated areas. There is widespread smoking in restaurants but, for the most part, people don't smoke while walking down the street and so you will find designated smoking areas with ashtray bins next to train stations. Take your cues from the people around you and you'll be just fine.
For the first time ever, we came home with money leftover! Japan has a reputation for being one of the most expensive cities in the world, but coming from another of the world's most expensive cities, we were pleasantly surprised. Firstly and most importantly, we had a really good exchange rate of about ¥184 to £1, which makes the world of difference, so if you can plan your travel flexibly, wait for a time when your money will go that bit further.
Secondly, I suppose Darren and I have cheap taste! We love eating ramen, street food and going to izakaya bars, but if it is your first time visiting Japan then it's worth splashing out a bit of money to experience proper restaurant dining or visit one of the bars in Tokyo with a great view. Once you get there, you'll want to see and do everything, and if you have the means to do so then you should go for it because if Japan is very far away for you, you're unlikely to be going back again soon.
If you want to experience some nice meals, but you're on a budget, go out at lunchtime. It's amazing how the prices can change for the same thing - sometimes half the price! Then you can spend your evening in an izakaya being a bit more frugal.
As a guide, here are some rough prices for food and drink (as of September 2015!)
Lunch set (tonkatsu, tempura, etc.) ¥1,700
Small plate in an izakaya ¥300 - ¥600
All you can eat yakinuku ('grill it yourself' meat) ¥2500 (you can find all you can drink too!)
Glass of beer ¥500 - ¥600
180ml serving of sake ¥800
Glass of umeshu/shochu cocktail ¥400
Soft drink ¥400