In Okinawa, you need to use Japanese Yen for purchasing by cash.
There are many shops accept credit card, so you don't needs cash for travel in general. conveniences stores (glossary shop), restaurants, drug stores, shopping malls, parking and toll road fee.
There is not tip system in Japanese restaurants.
However, still, some situations you need Japanese Yen such as local shops, coin parking, coin lockers, and coin shower.
Where you can exchange foreign currency to Japanese Yen
In Okinawa, you don't find exchange shop as New York or Paris. You can change at Automatic Exchange Machine (ATM).
You can find at Naha Airport and popular shopping mall.
An Okinawan big bank, Okinawa Ginko (Okigin) serve the ATM
see the map in their official web page attached to Google map. Otherwise, use translation function for your language because the web page is in Japanese.
Okigin goreign currency ATM pictures for your reference
Here is some pictures about foreign currency ATM, it is difference than cash withdraw ATM.
Here is front view of the ATM
Exchange fee is around 3%
This Okigin foreign currency exchange ATM takes around 3% as commission.
It is reasonable for me to compare to an exchange shop for small amount.
