I'm never without my gadgets! All this gear helps me recall and document my travels. I'm sometimes surprised at all the experiences I can recall when looking a photo or video of my travels. I just can't trust my mind to store the sensory overload that is Japan. It's also handy to be able to take picture of signs for later translation.
On the subject of translation, an easy-to-use electronic dictionary is really helpful both for interacting with people and for re-enforcing your Japanese language learning. If you're able to learn a new word and use it right away in a memorable experience you'll never forget it. Likewise, sometimes hearing that magic word will recall a special experience.
Some sort of PDA or Ipod can be really useful for storing a list of all the places you want to see, things you want to do, and things on your shopping list. It's also a convenient place for addresses and phone numbers - if you can get one that does Japanese. If it's equipped right, you can also use it at Wi-Fi hotspots to send quick emails - usually with something like "guess where I am now?" in the title.
A small GPS - once you get it set up with a Japanese maps can really help you find your way around in a country where the streets often have no names. You can also use it to "geotag" your photos. This really helps you to re-live your trip. Also you can use it to get back to places you want to return to on the next trip, later find out the name of something you see from the train. It's often much easier to snap picture than to set a waypoint on the GPS.
I usually rent a cell phone, but it can be expensive expensive.
Finally I need to carry chargers and power packs for all these devices. I'm a fan of AA size battery powered products, but there are some trade-off that I'll get into on a later post.