How many days shall I rebook seats

Hello,
I am travelling through Japan for 15 days. Starting in Tokyo, then travelling to Nagoya, Osaka, Hiroshima, Kyoto, Kobe and Kanazawa.

I am arriving at 2130 and staying in Tokyo for two nights before starting my travels. I wanted to ask would I need to book seats in advance or can I book on the day of travel? as I'm not sure if some of the journeys would be an overnight stay or day trips. Also how long is recommended to stay in each location? as I would like to experience some of the places at night time.

I have to return back to Tokyo on day 7 to visit friends and then I will be travelling around again from day 11 to 15 before heading back to the airport.