The Japan Railways Group has just announced a Seishun 18 Kippu special for the upcoming Spring 2007 season. To commemorate the 20th anniversary of the inauguration of the JR system (before that, it was the Japanese National Railways), the special-deal Seishun 18 Kippu will be offered at 8,000 yen (regular price: 10,500 yen), allowing, just like the regularly-priced Seishun 18 Kippu, five (5) days of unlimited travel on any JR futsu (regular) and kaisoku (rapid) trains along with a very small number of exceptions permitting travels on express trains (e.g., between Kanita and Kikonai sandwiching the Seikan Undersea Tunnel).

For the Spring season, you can buy the special-priced Seishun 18 Kippu between February 20 and March 31, and the pass can be used between March 1 and April 10 (last day of travel with Seishun 18 Kippu).

This is about the cheapest way to travel for five days in Japan that I have ever heard of, short of hitchhiking and/or walking. Granted that it can be tiring to travel far on a futsu or even a rapid train, but at only 1,600 yen per day, you don't even have to go the distance to make it worthwhile.