I've been there on a family vacation also some years ago. Stayed at Westin I think it was, beaches are beautiful and we had a good time. Rented a car which helps if you want to explore the island though there is not all that much to see.

Japan - Guam is a great mileage ticket. Its used to part of asia 2 for American but they moved it to South pacific. For Oneworld BA is still a good bargain, 20k avios and $65 usd one way in Business. For Star alliance I don't know all the ins and outs but I do know EVA air lists Guam as part of Asia (so its the same miles as for example ICN-TPE is ). There might be other good options out there also.