Hi,
I am planning a trip to Japan (this Friday!) and my boyfriend and I are ideally planning on hiring a car and checking out Mt Aso and the Onsens. we would like to also stay overnight in a Ryoken. How essential is it to book in advance? ...I know very little Japanese and wouldn't be able to call
thanks!!

It depends on the season and the destination! Some places always have availability until the last minute, while some villages are fully booked weeks in advances.

Now is pretty low season, it might be easier especially if you intend to stay in a ryokan during the week. But a place like Kurokawa onsen, for example, tend to be very popular all year round. Advance reservation would be a must to stay there.

Hello Emma,

It is winter season and Ryokan with Onsen are pretty much full, I will really not advice to try to go there without any reservation if you want something with nice bath.

To reserve have a look at this post : How to use Jalan and try to use it.
You can also use it and you will see the availability of the different hotel/ryokan.

To help you choose, you can always check our website for onsen : Secret-Japan / Onsen.

Bye

PS I edit your title and change Ryoken to Ryokan, this will be better when using the search engine later with more appropriate writting (旅館　りょかん　ryo ka n)