I've always liked Siesta Key Beach in Sarasota, FL. The sand is quartz & it looks like powdered sugar. However, if you want to surf or enjoy big waves the east coast is a better choice. The west coast generally does not get the bigger waves.
There are some lovely quiet beaches in the St Pete's - Clearwater area, plus a lot going on in St Pete's itself and in other parts of the beaches. Try Fort De Soto (St Pete's) or Caladesi Island (north of Clearwater), and check out www.floridasbeach.com for all the details.