Question about hostels/hotels in japan

I will be travelling to japan in a week with a couple of friends of mine, but none of us are very inclined to plan every step of our travel, we rather find out what we want to do once we are there, as we might enjoys some places more than others and might want to spend an extra day or two or none at all. That is why we can't book hostels/hotels in advance and my question to you is:

Can you drop in to hostels and find room? There is 3 of us so it shouldn't be a problem, will the charge differ? Are the hostels in japan crowded and should we order in advance? We will be flying down to tokyo first and we each have a budget of 120-150k (yen) give or take.

Any other suggestions such as things to do or places to visit are highly welcome as well :)

That's how I used to travel successfully on the past. But I always made reservations a day in advance or at least a few hours in advance.

Can you drop in to hostels and find room?

It is uncommon but not prohibited. It comes with the obvious risk of the hostel having no vacancies.

Are the hostels in japan crowded and should we order in advance?

It depends on location, season and weekday. During busy times they can be booked out, in the off season, they can be completely empty.

I recommend to make reservations at least before arrival.

I have found that quite a lot hotels that you can book with on line allow you to cancel without charge if you cancel by at least 1 day before the booked date (some promo bookings may be non cancellable so check the conditions before hand). at least you have some flexibility if you want to cancel at the last minute (at least a day in advance)