What to do in Japan without very much money?

I will go to Japan for two weeks at the end of Feb 2008. I am planning to fly in to Osaka and stay with a friend for a few days and after that I don't know what to do. I know I want to get to Tokyo for the last 3-4 days. I am not very picky and I don't need to stay at any expensive hotels. Actually I could even sleep in a cardboard box if I had to but I want to see as much as I possibly can. Any suggestions?