I will choose going to Okinawa. In late July, I went to Okinawa with my 9-year-old daughter. She enjoyed the trip a lot and said she wanted to go there again. She loved the acquarium a lot.
I went to Okinawa with my kids (7 yrs and 2 yrs) in end of July. They enjoy the trip very much especially the Ocean Museum and underwater boat. They love the sight seeing and foods. At the end of trip, they asked when could they come back here again. I think the radiation impact is comparative lower than Tokyo. Okinawa is around 1600 km far from Tokyo.