Athens is a very nice destination for a city trip. It has many interesting places to see during the day, nice beaches, good restaurants and exciting gay night life in the weekends at night. You can easily combine it with visit to Greek islands.
Mykonos is the most popular Greek island for the gays. It looks amazing, all houses are painted white, it has great beaches, nice fancy restaurants and gay night life. Mykonos is visited by many hot Italian and French guys.