I highly recommend St. Pete Beach! We go nearly every year for at least 1/2 of our Easter break. This year I only get 1 night in St. Pete thanks to the way too tempting 4/3 deal Disney is offering.
We love the warm gulf water, beautiful white sand and sunsets! Plenty for the college age kids to do ---but not spring break craziness on the beach (ie. drinking and 1/2 naked people.) Excursions like day trip to parks for snorkeling, dolphin cruises, parasailing and jet ski rentals on the beach. Great shopping at the Pier and St. John's Pass-not too far away)
(Wow-I'm starting to sound like the tourism bureau!)
Not sure when your dates are but I can tell you at Easter it is not cheap! (Although I have scored some good deals over the years-through priceline bidding and hotelkingdom.com.) At other times of the year, however, prices are very reasonable.
Hope this helps. Feel free to PM me if you'd like any other info or have questions.