Since you want to attract Japanese tourists, I assume for the relevance of the results you do not need submissions from other foreigners?
Furthermore I advice you to take a good look at the tripadvisor downloadable offline apps (Tokyo, Bangkok, Paris, Brussels, etc). I often use them when traveling, and they provide everything that you want to do for Guam. However, it could be improved with hotel/restaurant/venue reservation and booking possibilities, or an integrated navigation system.