On my second day on Okinawa, I spent the day on Kuroshima, an island about 30 minutes by ferry from Kohama.
We rented some bikes and rode around the whole island, which is famous for having more cows than people (2800 cows and only 216 people live on Kuroshima).
There were many beaches but unfortunately most of them were rock and therefore we weren't allowed to swim there. We did eventually find a sand beach and spent the last hour swimming there!
Here is some information about Kuroshima in case you are looking for a vacation spot in Okinawa!