Since I live in Florida (and my hubby and I don't mind driving a bit!) we did Spring Break weekends in Fort Myers, Daytona Beach, and Panama City (Fort Myers and Panama City for '01, Daytona and Panama City again for '02)
I would have to say my fave place was Panama City. It's a blast, there is a ton of people there...and Club La Vela rocks!
I live kinda close to Panama City (only 2 hours away) so we didn't stay overnight there or anything, just drove up for the day to party it up on the beach. It's so great!