I agree with kjazz. Check to see if they are in bloom. Sakura is simply beautiful.

In the Tokyo area, it usually doesn't come to full bloom til the first few weeks of April. Then again, it depends on how early or late it starts getting warm. It works its way up from the south and it doesn't last very long.

There are many good spots to go to. Ueno Park would be filled with people. There are many other parks also. Alongside the Meguro River is a good spot also. I experienced my first real "hanami" last year, drinking & sitting under the trees with friends. It was still a little chilly out but it was a very beautiful and fun experience.