Just wondering... but what has your father expressed about his travels to Japan?
Assuming you do plan on going. I'd suggest if you travel alone and not as a part of a guided tour, then planning an itinerary will be astronomically important to you.
Setting aside the notion that the itinerary will help you frame your budget, it's just my POV that having a travel plan and trying to stick to it will ensure you get out of your hotel each day. If you're the type who'd end up staying in their hotel rather then venturing about, then I'd say then that it might be considered a wasted trip.
Also in planning your itinerary, the research also becomes useful too. You learn basic geography, history lessons, cultural pointers, etc. that when you're actually in the country will make you feel more "clued in" to your surroundings instead of the fun killing "what the heck am I looking at/doing here?" quandry.
I hope you get to go and have a great time!