Key Biscayne enjoys a laid back island-like atmosphere, just minutes from downtown Miami. It's Crandon Park beaches are usualy sparsely populated and include restrooms, picnic, and playground facilities. If you're into snorkeling, you can snorkel from shore (300 yards out without a dive flag) and find some small reefs with lobster and tropical fish.
There's a $1 toll to get on the island and parking is $5.