Concierge.com's insider take:
Beaches in seaside cities often have an outsize influence on the culture. As in the south of France or Rio de Janeiro, the pull of the beaches can be found throughout Miami in both attire and attitude. Miami Beach's prime spit of sand is Lummus Park (though nobody actually calls it that), bordering Ocean Drive between 5th and 15th streets in South Beach. All the locals' gym time is shown off here, and, yes, there is plenty of topless bathing. Rent a chaise and umbrella from a concession, or just sprawl on the sand (the white stuff's not local, but shipped in regularly from the Bahamas). The area around 12th Street is an informal gay enclave; the rest is mixed. If you want to hang out with expat Latins, head a little further south to the beach around 3rd Street, popular with bikini-sporting Brazilians and Venezuelans. Miami Beach north of 15th Street is less scene-y, but the sand is just as luscious.
If you're looking for a more family-friendly strand, skip overhyped Sunny Isles (north of Miami Beach). The sand is wide there, but an ill-conceived beach renourishment project has significantly worsened the riptides in the water. Instead, hit Crandon Park Beach on Key Biscayne, another barrier island just south of Miami Beach but accessed via downtown Miami ($1.50 a car); the beach is just as wide, but a sandbar just offshore actually reduces lethal undertows. Also on Key Biscayne is Bill Baggs Cape Florida State Park, which has restaurants and a lighthouse—109 steps to the top (305-361-5811; www.floridastateparks.org/capeflorida).