Best time to Okinawa

We want to avoid the summer crowds in Okinawa so was thinking of going in Sept, will it be a lot quieter, hotels easier and is the weather sunny in Sept and still warm enough to swim ? Apart from Naha, would like to visit Miyako and Ishigaki islands.
By the way, can you rent a car on Miyako and Ishigaki and if so, is it a lot more expensive than say Narita (since we don't speak Japanese so difficult to find English car rental websites) ?

September is not bad except for the high chance of a typhoon interfering with your holidays.

I recommend late June and early July when the weather tends to be very sunny and the school summer holidays have not started yet. Alternatively, October is very nice.

The air and water remain warm enough for swimming in October.

By the way, can you rent a car on Miyako and Ishigaki and if so, is it a lot more expensive than say Narita (since we don't speak Japanese so difficult to find English car rental websites) ?

Yes, there are lots of car rental outlets on Miyako and Ishigaki. Some of the have ENglish websites:
http://rent.toyota.co.jp/en/index.html
http://www.nipponrentacar.co.jp/english/index.html
http://www.mazda-rentacar.com/

And here is a service that let's you make reservations through an English online form:
http://www.japan-guide.com/ad/coocom2.html

Kenny,

On Miyako-Jima I highly recommend you book the car hire in advance, perhaps through your accomodation. We arrived on Miyako-Jima in mid Sept. '11 to find ALL rental cars already booked out during our stay! :( It is very difficult and inconvenient to travel without a rental car/motorbike on MiyakoJima!

Stew

On Miyako-Jima we found the cost of car hire much cheaper than the rest of Japan (about Y4000/day in some places) however the cars were significantly older.

On Ishigaki we found the cost car hire more expensive than the rest of Japan.

Thanks Uji and Stew, your answers have been most helpful. Any comments on which rental company in Okinawa is good / not so good in all or any respect ?
Just started planning for our trip in Oct but seems to me not a lot to do/see on the main Island (if we save the beach visits for Miyako and Ishigaki). Been to the aquarium in Osaka and elsewhere, so unless there's something special about the one in Okinawa, would skip that. And with a car, I would say could see the main island in a day, any suggestions ?

I haven't been to the aquarium in Osaka, but the one in Okinawa has a massive tanks with several whale sharks and other creatures swimming around... http://www.youtube.com/watch?v=u7deClndzQw

Thanks Smithers, if we have time will check out the Aquarium because the layout is rather interesting.
What else did anyone find interesting from their experience to do/see (apart from the beaches and aquarium) as it seems I could cover the main island in a day with a car from just looking at the ususal tourist sites ? I ''feel'' there ought to be more to Okinawa.

The festival in Naha will be held on 6-8 October, 2012.

http://www.okinawastory.jp/en/event/event_111008-10.html

Our original plan to Okinawa this October fell through, so have to postpone to 2013 either April, May or June--what time would be best ? Thanks.