Check to see if the cherry blossoms are out yet. There is a lot of partying going on in Ueno park as people reserve seats under the rows of cherry trees to take them in and to really get out to drink and eat. The park is at Ueno Station off the Yamanote line (green train that circles Tokyo and stops at most major stations). If you bought a JR pass it is good for 7 or 14 days and you can start it at Narita airport to take the $30 train ride into Tokyo. If not take the slow train for $10. It's easy to spend $10-15/day on trains and subways so see if it pays to buy a day pass. There are only a couple of JR lines in Tokyo to use the rail pass. You really need to go to another city to pay for the train pass. Break even is making it to at least Kyoto and back to Tokyo. You better do some research or be on a tour to get the most out of it. Start here: http://www.jnto.go.jp/eng/index.html

Stops in Tokyo:
Harajuku on a Sunday for the costume kids, Yoyogi Park on a Sunday to see the rockers, Shibuya shopping, Shinjuku clubs, Odaiba cool reclaimed island, Roppongi clubs and shopping, Ueno shopping under the tracks, Asakusa Senso-Kannon Temple, Sony Building, Akihabara for the electronics shops.

Kyoto is great for temples and history as it was not bombed during WW2. Many many world famous sites there.

Hiroshima - A-bomb museum.

My favorite thing to do is to go to an onsen for a little relaxation.