I put Toyooka here but I actually mean specifically "Kinosaki Onsen". It is absolutely my favourite onsen in Japan. It has a fantastic traditional atmosphere, and if you get lucky enough to come here in the winter when it snows, it's absolutely stunning.
Getting to Kinosaki Onsen
There are three main ways of getting there. By car, train, or bus. I've never gone by car so I can't speak on that. It's the most comfortable to go by train, but it's cheaper to go by bus. If you have a JR pass, the train is covered. I think it's about 3-4 hours by bus and 2-3 hours by train? The train can either go to Osaka or Kyoto from Kinosaki Onsen.
It's pretty pricey to stay at a ryokan, but it's a must-do if you come to Japan. Book one of the ryokan here, and you'll get a pass that gives you access to all the different onsens in the town. You check in around 3pm usually, go out to soak in some onsen, go back to the ryokan for dinner around 6, and then you can go back out for more onsen. Every onsen has different features! Some have outdoor rotenburo, while others are like a cave. For those who love onsen, the pass is valid until the next day so you can wake up early and go to more onsen before breakfast, or even check out (and store your luggage at the ryokan) and come back after you've gone to a few more onsen.
They're famous for their crab courses in the winter, but I would recommend trying the traditional courses if you come from a country that has a lot of seafood (and you eat crab often anyways). The crab is tasty but if you're only staying one night, go for the Japanese experience.
Things to do
If you're not interested in trying out too many onsens, there's also a mountain with a cable car that you can explore. You can hike it instead of taking the cable car, but it's really quite a trek and was muddy when I climbed it once in late February one year.
Leaving Kinosaki Onsen
We headed to Tokyo straight from Kinosaki Onsen (transferring in Kyoto to the Shinkansen). Note that the buses and trains come rather infrequently so you should research the timing in advance, or ask your ryokan staff to help you book.
Back to Tokyo
See the Tokyo section to continue the trip on Day 8.