In the Terminal
Set the indicator to show extra locations.
gsettings set com.canonical.indicator.datetime show-locations true
Set which locations to show. Timezones are in the format Continent/City_Name City Name
gsettings set com.canonical.indicator.datetime locations "['America/Los_Angeles Los Angeles', 'America/New_York New York', 'Europe/London London', 'Europe/Paris Paris', 'Asia/Tokyo Tokyo']"
To undo your changes, you can simply hide the locations again.
gsettings set com.canonical.indicator.datetime show-locations truefalse