I would really suggest booking hotels in advance. The kinds of hotels you can book at the last minute probably aren't going to be "vacation of a lifetime" type hotels, especially for three people. I'm not suggesting there's something wrong with doing that for the people who do, but you're probably not going to get the best deals or the best rooms.

I usually spend several months researching hotels and watching deals before pulling the trigger. My wife tells me I have a talent for picking memorable hotels, but it's really just a question of looking at what's available, narrowing it down based on whatever criteria you have, then setting a pricing strategy and waiting. You can get lucky by winging it and booking at the last minute, but you'll be just as likely to get an older, smaller room with no view in a hotel further away from where you want to be.

I think that makes it better to plan out the whole trip in advance, at least in terms of what cities you want to be in. You can still wing it with the specific places you want to go in those cities.

The rail pass also makes a lot of last-minute day trips easier. You can go from Tokyo to Nagoya and be back in time for dinner. So there's still a lot of opportunity for being impulsive, but I just think it's best to have your home bases mapped out in advance.