On a budget, Key Biscayne / Bill Baggs State Park still earns its price.
Park entry ~$8/car + causeway toll
The antidote to South Beach — calmer water, fewer crowds, a 19th-century lighthouse to climb, and skyline views across the bay. The catch: you need a car (and pay a park toll plus the Rickenbacker Causeway), and there are fewer amenities than the busy beaches.