You might also want to pick up a little travel guide book; Lonely Planet or Fodor's are popular choices. I always pick one up when I travel, they're so helpful! I'll be going to Tokyo (and HK) too in a couple of weeks!
Edit: If you're in Tokyo the first 2 weeks of April you'll catch the Cherry Blossoms in season!
Edit: If you're in Tokyo the first 2 weeks of April you'll catch the Cherry Blossoms in season!