Originally Posted by bmwe92fan

So much to answer -- but I'll give you a few things to think about:

1) The rainy season in Japan typically begins the first week of June and runs to mid July. It will be VERY hot and VERY humid. I have visions of the kids wilting (we are typically out of Japan in the summers due to this).
2) December is our second favorite month in Tokyo (we live here) -- humidity is low and the weather is typically sunny and pleasant -- but cool. Average high temps of around 10-15 usually - cooler at night.
3) Definitely take the Shinkansen at least once -- we use it all the time and love it as it is so much more relaxing (to us of course). Flying domestically in Japan is quite easy -- but don't forget the expense of getting from the airport to the city -- some are quite far away and taxis are not cheap.
4) Rental cars are pretty easy as well -- just make sure you have an international drivers license -- or a license that is accepted for rentals in Japan. Just a note -- parking in major cities can be expensive and sometimes difficult to find...
5) Pack as lightly as you can -- the more luggage you have the more of a pain it is to get around...
6) On the hotels -- see if you can find ones with "family" rooms -- or book a suite if possible. There are hotel rooms that will accommodate 4 -- but perhaps not available for award travel -- and yes they are a bit harder to find -- especially at global chains like Marriott...
7) Renting an apartment is also an option -- if you go this route I would suggest you determine the sites you want to see and then figure out where to find an apartment. We live in central Tokyo and our building does not allow any short term rental apartments (MANY buildings have this rule). I won't go in to all of the reasons why in Japan things like AirBNB are not as popular -- but they aren't as plentiful or as well located as they are in other countries around the world.