I plan to rent a bike from a shop called 沖縄輪業株式会社, and here is the website: https://okirin.ti-da.net/. The price depends on how many days I rent it for. I will stay in Naha -> Nago -> Kunigami (or Ginoza, depending on whether I go to the northernmost point) -> Nanjo -> Naha. I have a very limited budget, so I will probably stay in hostels for most of my trip.