I found out that there are some kinds of events held on land that is part of US bases here in Okinawa that are attended by the general public, Japanese and American.
The particular event I noticed was a weekly flea market each Sunday.
I know it's open to the local Japanese as well as the US military. But I don't know what the rules are for people of other nationalities. Are passports checked? Would non Japanese need to have a US visa?