1. The Basics
Toire (トイレ) = toilet
Washiki toire (和式トイレ) = Japanese-style toilet
Yoshiki toire (洋式トイレ) = Western-style toilet
If you want to know the details of using a Japanese-style toilet, have a gander here. They're not very common anymore, but you'll still run into them in public schools (hello, ALTs!) and older train stations.
O-tearai (お手洗い) = washroom (more polite; lit. hand-washing)
Keshoshitsu (化粧室) = restroom (very polite; lit. make-up room)
Benjo (便所) = toilet (less polite; lit. poo place)
Kawaya (厠) = toilet (archaic)
Toire is fairly common conversationally, but you'll tend to see the term o-tearai used in most casual signage (be aware that the final "い" character is optional!). While keshoshitsu was once more of a term for high-end restaurants and hotels, it can also be seen in newer train stations as they try to make their washrooms sound classier. Meanwhile, some kids' games—such as Hankachi Otoshi, the Japanese equivalent to Duck-Duck-Goose—will gleefully name the loser's place of shame the benjo. And if you mention a kawaya, you'll probably turn a few heads by saying a word that even most Japanese people don't use anymore!
Toiretto peh-paa (トイレットペーパー) = toilet paper
Benjo surippa (便所スリッパ) = bathroom slippers
Soh-pu (ソープ) = soap
Hando doraiya (ハンドドライヤー) = hand dryer
A lot of those words sound familiar, don't they? They're examples of gai-rai-go—foreign words that have been appropriated into Japanese. As per tradition, they're written in the angular katakana alphabet.
4. In a Pinch
O-tearai wo o-kari dekimasu ka? = May I use your washroom?
O-tearai wa doko desu ka? = Where is the washroom?
Chotto o-tearai eh… = I’m just going to hit the washroom...
And if you're really in trouble, just say ”toire?” and look distressed—which should be easy enough!
Feel ready? Now you can navigate Japan's lavatorial wonders with ease!