Iriomote has a lot of nature. I recommend a rent-a-car so that you can get around efficiently.
Iriomote has 2 ports, Uehara port and Ohara port. Uehara is more convenient, but has more chance that the boat may be cancelled due to rough seas.
Some places to visit if you have only one day are Funauki, Mariudo Falls, Kampire Falls, Star Sand Beach and Yubu Island.
Funauki is not an island, but you can only go there by boat from Shirahama port. From Uehara port it is about 20 minutes by car to Shirahama port, then a 10-minute boat ride to Funauki, and a 10-minute walk to a beach called Ida-no-hama. It is said to be the most beautiful beach in Iriomote.
The boat time table (March 1 to Nov 30):
Leaving Shirahama 8：35、10：55、13：20、15：50 (July 21 to Aug 31)、17：50
Leaving Funauki 8：15、10：35、12：50、15：30 (July 21 to Aug 31)、17：10
Please check the latest information in advance.
Mariudo Falls and Kampire Falls
On the road from Uehara to Shirohama port (or going back), you can stop at Urauchi and take a boat up the Urauchi river (a 30-minutes ride), then a 30-minute walk to get to the viewing platform for Mariudo falls, and another 5-minute walk to Kampire falls.
Star Sand Beach
It is about 5 minutes drive from Uehara port, on the way to Funauki. It is not easy to find star shaped sand anymore. Can you find one in the below photo?
It is about 36 minutes by car from Uehara port, in the opposite direction from the destinations above. You can walk to the island or take a buffalo cart. There is a botanical garden there.
Just by driving, you can see some beautiful scenery. Just stop the car and enjoy the view. Also there are a lot of kayak and trekking tours. The sunrise and the sunset are beautiful. The stars at night are magnificent. You can also find many wild creatures. There was very little traffic when we visited in March. There were maybe two traffic lights on the whole island. Drive carefully – there may be animals on the road. A few iriomote mountain cats are killed every year and they are almost extinct.