We decided to do Okinawa World before heading over to Valley of Gangala. We enjoyed it and if you don't make the show times you can probably go around to everything in about 2 hours. My husband was surprised on how long the cave was at the beginning and that was probably our favorite thing overall followed by the Habu Museum. 

Would recommend doing at least once when in Okinawa. 
-you can get tickets there or online 
-staff friendly