I grew up in Tampa, so I do happen to know quite a bit about the beaches in that area. Now, first and foremost, I do not know the laws in Florida regarding metal detecting because I have been away for 6 years now, only going back when I take leave. I would definitely check that out before hand. There are plenty of options depending on travel range.
These are my suggestions based on the type of people that go to these beaches (I am assuming you do not want to find fishing weights every 10 feet).
Tampa/St. Pete/Largo area:
Ft. Desoto park. This is hands down the best choice for the local Tampa area. Traffic is constant during the beach season. You have a few different beach areas to go to here. North and South beach gets a lot of swimmers. Arrowhead beach gets mostly people digging in the dirt for arrowheads(could be good, rings slip off in mud!).
Clearwater beach is definitely another solid choice, however make sure that you don't get to frustrated with the trash drops/pull tabs. There will be many, I promise.
Sand Key County park is a fair choice as well. There will be much less trash then Clearwater. Also, there tends to be less of the teen/college crowd and more of the family/grandparents crowd.
Other beaches:
Dayton is not far, and with spring break just ending could prove promising. I would go there over crystal beach if that is an option.
Miami is 3-4 hours, depending on traffic and your speed. There is no end to the amount of stuff dropped at the beaches in Miami.
As far as what Matt said, I agree completely. Crystal beach is not really a beach. I would not go there unless you are going to visit relatives or just going to see the area.
I hope this helps, and best of luck to you out there! Be sure to let us know if you get anything!