Thank you for your desire to serve in the disaster zone in northeast Japan.
Unfortunately, there are few 1- or 2-day volunteering opportunities in the disaster zone that I know of. In general, most organizations require a minimum commitment of 7-10 days. There are local church efforts where volunteers head there for 1-2 days, but this is usually comprised of members of Japanese churches and missionaries who speak Japanese. The only organization I have found is CRASH/Japan (click here)
-- their website indicates that individuals need to serve at least two days, and teams for four days.
I'm not sure that there are many relief organizations still serving in that zone anymore; most have wrapped up work within the first 12 months of the disaster. I think this is true of Samaritan's Purse, for example. If there are orgs there, it's a little harder to find service opportunities for volunteers now that we've passed the first anniversary. Much of the grunt work/cleanup work has been done by tons of volunteers from inside Japan and outside, and now the majority of displaced people are living in temporary housing units. The government has been rebuilding the infrastructure mainly. From my limited vantage point, what is really needed now is for the long-term rebuilding process of people's lives and communities. Vision for this movement is actually being led in part by Japanese local churches across the affected area. While the need still includes supplies and some labor (e.g., construction, rebuilding), it mostly means emotional support for people, as well as long-term community/economic development. I believe that local Japanese churches, with help from the business sector, will be able to do this best.
Short-termers/volunteers will not be able to contribute significantly to these community development endeavors. At present, we don't plan for the EnVision program participants to head into the disaster zone, because what is needed there is "practitioners" over the long haul, not "observers" for a week or two, if you will. The j-Teams program will likely run only during the summer months, and will aim to provide assistance to local church outreach programs.
I know this is much more info than you asked for, but I hope it serves as a trigger for you and others to continue to pray for Japan -- especially for the Church in Japan. In addition to minister through prayer for Japan, I trust God will guide you to the right opportunity while you are there.
God's best to you!