Travelling light is really helpful in Japan, due to the amount of time you spend getting on and off trains, and in and out of train stations.
I'm no packing saint, I define 'light' as one suitcase that I can heave around reasonably easily, and will fit in an overhead luggage rack if absolutely required (I have little faith in this perilous system). But there are things I've either forgotten and really missed, or brought and been eternally grateful for.
Here are my top picks:
In Japan, expect to walk much further than you think you will/can. The shoes you choose are important, but your socks are vital. I learned about moisture-wicking socks from the excellent Blister Prevention site, and they have been super helpful for Japan travels. Turns out there’s a lot of sock technology I didn’t know about, and some of it gets pricey. I like the Nike ones which look like regular socks, are cheap ($20ish) for a 3 pack and come in different lengths. As a bonus, having nice new socks helps prevent intense feelings of shame every time you take your shoes off in public.
While this is probably one thing you could leave until you get there because Uniqlo is literally everywhere, I've always been very happy to have my HeatTech tees. We've tended to be in Japan in winter, when it's cold but everywhere is remarkably well heated. So layers are the order of the day, and HeatTech makes a great, cheap base layer. Chris swears by Icebreaker but, displaying a horrible lack of patriotism, I prefer the cheap and cheerful Japanese option.
On Trip Two I ran out of dental floss early, and this turned out to be a problem. It's not that Japan doesn't love a dental floss-style solution. I found loads of picks, and miniature floss head things, and what looked like tiny plastic swords, but very little actual floss. And certainly no Oral B Satin Tape. I... have a problem.
Im Melbourne, I don't usually carry a backpack because of the amount of time I spend on trams and the associated amount of time I've spent with someone's knapsack obstructing my airways. My love forever to the nice people who take them off when they get on board. But in Japan, especially when carrying my light-ish suitcase in tow, a backpack is great. Not only will it hold all the essentials, including a raincoat, WiFi and batteries, camera guff and a collection of 7/11 snacks, but when you inevitably wind up with a plastic bag of rubbish, you'll be glad you have somewhere to stash it.
Pack in your hand luggage (I think? Don't quote me, this seems to change all the time) and then thank yourself the entire time. Between two phones and one portable WiFi, there's always something that needs to be charged. It wasn't just the times that we planned to be out for a longer time either, I've found that in locations where the signal is weaker, the portable WiFi will run out of battery a lot faster. If you really get stuck, Lawson and Family Mart sell pre-charged units, which also plug directly into the wall when you need to power them up again.
It was Trip One when we decided that we'd just "grab a couple of adapators in Japan". This was a terrible idea. I'm sure they're possible to find, and I'm about to get told to "head to Bic Camera", but we never quite managed. And I've never made the mistake again. We now have a bag of random adaptors and multiboards that get passed around to people before they head across.
There is no shortage of great coffee in Japan. I even wrote about it! But it's not always on hand when you want it, which for me, is 8am every day. The AeroPress is the best way to take the urgency out of finding that first cup, and keep travel tempers even. Plus, there are loads of small roasteries in Japan to buy great beans from.