We've done Girls Weekends away at the Spa at the Hotel Hershey.....it was wonderful! Very laid bacck, though....no nightlife. But there is a very nice restauarant in the hotel.
The Borgata in Atlantic city also has a great spa, good clubs, and nice restaurants. Lots to do just right in the hotel/casino/spa itself.
If you want a spa type weekend in NYC, some of the hotels have spas, too. but doing the shows, shopping, and a spa in one weekend is alot to cram in one weekend in NYC!!!