There's not much in Naha, it's just a standard modern Japanese city. Maybe one day for shops, etc. There is a small beach, but more of a city beach than somewhere to spend days on.
There are some war sites in the south of the island and a big museum. You could bus to there and do some walking. Good coastal scenery.
From Naha you can get ferries to some outlying islands. Some have beaches and good scenery and make good day trips. I don't know if they have hiking trails as such, but they looked good for walking. You can stay on them if you want, but I imagine they are pretty quiet once the last ferry has left.
I found the best beaches Okinawa were around the Nago Peninsula, particularly the North side. There is fairly good bus service on the island, but for these beaches you would want to stay near to them. You will have to research accommodation.
There were also a few beaches between Naha and Nago with resort type hotels. Again you will need to research.
Once about 40 kms north of Naha to the top, the centre of the island is hilly with forest cover. Few roads or settlements. Good wilderness stuff, if you are in to that.