[dirtyroboto]

Most in car GPS have multi lingual settings. You may have to ask the rep to switch it to English for you.
That said, all of the place names and text searches will still be in Japanese on systems loaded with east asian maps.

You can always use google maps/earth and note down the co-ords (before you arrive here) then type them in manually.

Please ask about ETC charges (toll roads) to find out what you are liable for. Please avoid entering an ETC approach road, it is one way, you will have to pay to travel to the next exit just to get back off.

When hiring a car here, look for the most fuel efficient. You can spend allot of $$ on fuel which is about ¥125 - ¥130 per liter