What options are there in terms of prepaid Internet services? Are the charges reasonable? I will be there for no more than two weeks so prepaid seems to be the sensible option.
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There are two good options.
If you're OK with relying on Wifi hotspots, instead of needing 100% coverage, the best option is to sign up for a Wi2 300 monthly account. (Available in Japanese only, and not to be confused with the far more expensive travel plans for tourists!) For Y362/month, you get access to their own hotspots plus the Yahoo!BB and UQ networks, for a total of over
70,000 200,000 access points around Japan. (See FAQ.) They provide a "Wi2Connect4" app for iOS devices to ease the pain of typing in network IDs etc. And yes, you can cancel with no strings attached once you get home and pay only a single month's fee — at time of writing they even have a "first month free" promo.
That said, while the number sounds impressive and the map looks packed as well, you can't count on having a hotspot whenever you want one. In particular, the "premium" hotspots (Wi2Premium, orange on map) incur an extra charge of ¥100/hr.
If you want actual 4G mobile internet on your cellphone available anywhere, the cheapest reasonable option I'm aware of is the B-Mobile 1 month, 1 GB prepaid, but it's a lot pricier at Y3450 and a little harder to find, although Yodobashi and Bic Camera both stock it.