A ryokan is a traditional style Japanese inn. They offer great service, traditional food and the chance to sleep on the floor. A couple of pointers will avoid a spot of embarrassment. The shoes-off-at-the-entrance part is pretty easy.
It's the slippers that can be a challenge. Never, ever wear slippers on tatami , the straw mat floors in the room. And for the biggest "oops, I'm a foreigner" faux pas, make sure you don't walk around wearing the slippers that say "toilet". They are only for the toilet.