As for the beaches: you'll generally some topless beachgoers, but not completely nude. There is one section at the far end of Orient Beach that is clothing optional, but we haven't seen any widespread full nudity on the St. Martin beaches.

Orient Beach is gorgeous, and always has a nice breeze. There are lots of other, smaller beaches you'll want to try, too.
Where are you staying- Dutch side or French side? The Dutch side is a lot more Americanized.
The French side has some wonderful dining. You can't go wrong visiting Grand Case for dinner. The biggest problem there is deciding WHICH of the incredible little restaurants to visit for dinner!
We like to visit the little bar at the end of the runway at the airport to drink a few beers and watch the planes come in. (Sunset Beach Bar?)
We like to visit Philipsburg and roam the shops and watch all the people getting off and on the cruise ships.
In Grand Case, Calmos Cafe is a laid back place where you can eat a burger on the beach while you dig your feet into the sand and watch the sun go down.
If you tell me where you're staying, I can tell you our favorite spots around there.
