The Japan Rail Pass allows for a lot of spontaneity, since all trains, including the Shinkansen, have at least some, if not all, non-reserved seats.

I've even changed my mind about my destination in the middle of a trip.

However, since you have five months to plan, you have time to buy a guidebook and study it carefully. Japan is small, so you can cover a lot of ground in eight days.

The standard tourist attractions outside of Tokyo are Kyoto, Nara, Hiroshima, Hakone, and Nikko, and those would make for a full eight days.

If you're staying at the American Club while in Tokyo, then you are not budget travelers, so it will be hard to go wrong on hotels. I've used booking.com in Japan, Korea, and Scandinavia, and I've found the users' ratings and photos to be accurate. It has hotels in all price ranges. The best thing about booking.com is that it allows cancellations up to 6PM local time the day before your scheduled stay, which is about the most spontaneity you can hope for.