Regarding your last point, it's always a good idea to bring a compact power strip with you when travelling. Something like this or this, though you may need a 3-prong to 2-prong converter for some places in Japan. They are great for nights when you have to charge multiple devices and make you a hero at airports.
One thing I forgot to mention was what I packed. Obviously being a guy some of the stuff will be different but I just wanted to do a quick run down. It's not everything but a highlight of what was amazing and I'm glad I packed, and what was just taking up space.
Sandals. Something you can wear in the shower. This is a MUST as showers in hostels (much like dorm showers) are usually not something you want to be barefoot in, or at least not in the bathroom/going to and from the shower.
If you're going to spend $18 on a pair of socks, check out darn tough. They come with a lifetime replacement warranty. I hear stories about people who have dogs that eat them and the company still replaces them. While I don't currently have any, (I got a bunch of Cabela's merino socks on clearance) I do swear by merino wool socks for some things (hiking, cold weather, etc). Just one example - amazon link - $18
TSA Locks. Hear me out. These are not for real security. They are however to slow someone down/discourage someone. Good while traveling on trains or planes. Not a necessity, but came in handy a few times. I like the combo ones so I don't need to worry about having a key. Amazon example - $7
Cable lock. This was the other lock I got. I'm glad I did. A couple of hostels didn't have lockers in the rooms (only a small metal lock box for passport and camera). What I then did was lock my backpack to the frame of the bed so it wouldn't walk away. Amazon - $13
The locker lock. You'll notice all the locks are combination. I don't like carry keys when I'm on vacation. I tend to lose them. Might just be a me thing. This is the lock I got. Might have been over kill but I teach people how to pick locks, so I wasn't going to get a cheap padlock. Amazon - $23
Fast dry towel. By far one of the best purchases for my trip. This will save you money (Hostels often charge to rent towels) and is a lot smaller and quicker to dry than a regular towel. I found washing it once or twice stopped it from shedding on me when I used it. Amazon - $10-20 depending on size
Face mask for sleeping. I literally used it once. Wasn't worth the $8 I spent for it. For you it might be different. Amazon - $8
You know what else I almost never used? Ear plugs. I think I got lucky and most of the hostels were pretty quiet. One I stayed it was over what was basically a night club. That got kind of loud. Amazon - $7
Packing cubes. Since I was bringing some nice shirts and pants this came in handy. Since you probably wont need the garment folder I'll link the cheaper 3 pack too. Some people prefer to not use these. For me it helped keep things organized and stopped me from loosing things. More expensive one - $44 Cheaper one with out flat garment folder - $27
Sink laundry kit. I used this more than a few times. In fact, in a month I only did laundry in a machine twice.... That's with only having two pairs of pants too. Most of the time I washed my shirts, boxers, socks, and even pants in the sink or shower with me. Which leads me to the next thing I got.... Sink laundry kit - $8
Clothes line. Let me say, the suction cups on this were useless for me. Put any weight on it and they didn't hold. Hence the clips at the ends came in handy. I also had a few girls ask if they could use it to dry their clothes. Clothes line - $9
Flashlight. If you always have your phone on you, and know it'll always be charged, then this might not be an issue. For me I love having flashlights handy. I make sure my mom has one of these attached to the zipper of her purse. Comes in handy. Amazon - $12
Travel power adapter. I wanted one that was small and had a built in usb charger. This fit the bill. I like that it meant I didn't need to bring separate usb chargers. Amazon - $17 not Prime, $22 Prime
Collapsable 1L water bottle. Make sure you bring a water bottle. Planes, trains, and really everything is better if you have a water bottle. I got two and once I got through security I filled them up at the airport. Amazon - $10
Travel power strip. I literally NEVER used this. Waste of money for me. Your milage may vary. Amazon - $29
Activated charcoal shoe anti odor bags. I figured if I'm going to wear the same pair of shoes for a month in the middle of summer this might come in handy. I would also throw one in my dirty clothes bag and backpack to keep the smell down. Probably won't be as much of an issue during the winter. Amazon - $10
Cologne mini bottles. So you keep the smell down... now how do you smell good? For me there was no way I was going to bring a big bottle of cologne. So I got these instead. Amazon - $8
Lastly on this insane shopping list of things you totally don't need to buy, but might be good suggestions/ideas is... a notebook. I prefer moleskine personally. You said it yourself. This is the first of many trips. You're going to see a lot and you're going to want to come back later. Make sure to write stuff down. Journal every day if possible. I got close. Amazon - $16
Hope that helps!