I visited Ishigaki in December 2019. While I enjoyed it, without a car you'll be limited for options. I went on the only bus tour of the island available - it was in Japanese only but was still enjoyable. Searching now it doesn't seem to available to book anymore, perhaps it no longer runs. There are private taxi and boat tours, though. Ishigaki is a good base for trips to Taketoki and Iriomote if you want to try to cover as much ground as possible in a short period of time.