It used to be that if you were going to more than two destinations in Japan (I e. Tokyo + Kyoto + Somewhere else) your best bet as a foreigner was a Japan Rail Pass which needs to be purchased in the US before you go. I don't know if that math still works but worth investigating. Assuming you aren't flying to Kyushu or Hokkaido
Even casual/ kid friendly places will require booking. Not necessarily days in advance, but walk-ins are hard.
Tokyo is wonderful. But a lot of what makes it amazing are things that won't appeal to teen boys (dining, bars, walking neighborhoods checking out local retail/cafes etc) Make a list of what sights you actually want to see and figure out how many days you need. 7 days might be a lot. If you did 5 in Tokyo 2-3 in Kyoto/Nara and then 2 more some place else you'd have a pretty great first time trip.