Before we begin, know this: I've lived in Tokyo only a few months, and before that I visited a handful of times. So this is by no means the definitive guide to visiting the city; it's just what's worked for me.
That said, Tokyo is fucking amazing and you cannot help but have a blast. Questions? Feel free to contact me directly: firstname.lastname@example.org
Okay, so you're coming to Tokyo? Rent a pocket wifi. Why?
1. Paper maps and guidebooks will be beyond useless here (see #2).
2. I hope you're a smartphone user and you know how to use a map application here. You'll need one to not be lost all the time, every day. The streets here are not named in the way we from the U.S. think of street names, and the addressing system is so arcane that even some lifelong Tokyo residents don't quite understand it. Plus, the streets are so numerous, narrow, and winding that it's insanely easy to get lost. I've gotten lost in my own neighborhood barely 300 feet from my apartment. Ever been to London? It's like that, but way more confusing.
3. Free wifi is nearly nonexistent here. You'll find it in McDonald's and Starbucks (yes, coffee lovers, there are loads of Starbucks here), but in order to access it, you need to sign up at home. You can't sign up on the spot. Why? The Japanese thrive on bureaucracy, I guess.
4. Sometimes, you can't even rely on your hotel for free wifi. The first time I came, I was in a glorified hostel that had one ancient PC running some creaky old version of Windows, and it cost $1 for every ten minutes of use. I've also been in hotels that had wifi only in the lobby, and only ethernet cables in the rooms. Not so useful for the iPad.
5. I've always rented a pocket wifi from globaladvancedcomm.com. It's not terribly expensive, and you'll get a small discount for liking their FB page. Also, you can have them send it to the post office at the airport and pick it up before catching the train into the city.
6. With your groovy pocket wifi, you can leave the bulky guidebook in the room. Make your own custom Google Map with all the places you want to visit, and wander freely!
7. If you have T-Mobile, you will have free international data. I've got T-Mobile, and I find that it works in a pinch, but it's only 3G, so it's slow. Fine for emailing and Twittering, not great for Instagramming and Facebookery.