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.
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.