I lived in Miami for 3 years - Shake Shack is a NY transplant semi-fast food joint, but it's soooo good - it's right off Lincoln Rd. Their burgers are amazing, but their key lime concrete (a frozen custard) is to die for!
Best sushi ever is a dive bar up on 71st (72nd? I forget) called Katana. The sushi goes by in boats, but let this girl tell you, it's amazing sushi! And rock bottom prices - my husband and I would eat our fill, and have beers & sake, and never spend more than $45 for the two of us. You'll find that nowhere else on Miami Beach.
For a great fancy night out, call ahead for reservations at Meat Market on Lincoln Rd. Very, very good steak! Pricey, but worth it.
Also, it's still stone crab season, but they've had a poor harvest this year so it might be more expensive, but check out Joe's Stone Crab (again, pricey, but soooo yummy!).