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 account, which has a friendly English signup page and does not choke on foreign credit cards. For Y380/month, you get access to their own hotspots plus the Yahoo!BB and UQ networks, for a total of over 70,000 (!) 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.
If you must have 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.