[Sangetsu]

Current 3G iPads in Japan use the Softbank network. Normally the only way you can get temporary service is to rent a phone or a SIM card. Since you can't change the SIM card in an iPad easily, you are probably out of luck.

I use a wi-fi iPad with a pocket wi-fi modem, and it works well. Wi-fi is available just about everywhere, so it's probably the best option for you.