It took more than three trips to perfect 3 days in Miami, but after traveling there with girlfriends, by myself, with family, with a boyfriend and with work I think I've got a Miami for everyone:
Head to the beach first thing in the mornings--the sun is not at its full heat and it's surprisingly empty until around noon so you can take the best spot for yourself.
For lunch, head to Burger Baby on Washington and 15th. They have a perfect outdoor cafe hidden in the back complete with a tiki bar and hammock. Their fresh cooked burgers and fries are perfect to pair with a beer after a day in the sun.
The rest of the afternoon can be spent on Lincoln Road shopping, smoking hookah, and taking in the sunset on the other side of the island. For a local (dive) bar experience, head to Lost Weekend. It's right across from Señor Frogs, which provides a comical contrast to window watch.
Another beach day? Why not! I would recommend some SCUBA or snorkel adventures in the morning. There aren't tons of SCUBA options in South Beach, but the ones that are there provide regularly scheduled trips. I used South Beach Divers who were helpful and professional, but maybe not so great at communicating plans (as in not mentioning that the van you're going to isn't stopping for two hours until we get to Key Largo, for instance). You can reserve a spot directly online for a discount.
From South Beach it's possible to take a bus or taxi to the Perez Art Museum. Their cafe, Verde, has amazing lunch and snack options. The modern and contemporary art comes from all types of artists from all over the world, and is a perfect 2-3 hour collection to wander and ponder through. It is located next to a park over-looking Biscayne Bay and is worth spending some time at.
After that, enjoy a perfect dinner at Mignonette, a converted gas station that now serves the best oysters I've ever had. All of downtown Miami is kind of awkward to walk--people will look at you funny--but it's totally possible and not very far at all. It helps that this area is where the famous Wynwood street art begins, so there's plenty to look at. From Mignonette you can walk to the Wynwood Walls and admire modern art, from graffiti to collages. I would recommend not sticking around too late (especially if you're traveling alone), and to simply take a taxi or Uber back to South Beach. If you're feeling classy check out the Alice-in-Wonderland evoking Delano (drinks are $20 each) and dance in the ankle deep pool. If that's not in your budget, dance the night away at any South Beach club.
While my hostel offered free breakfast every day, there are plenty of options to enjoy in the morning (like the Front Porch Cafe or A La Folie). If you're sunburnt and ready for a break from the sun, are plenty of cool spaces in South Beach to check out, from The Chill Concept (a pop-up museum) to the World Erotic Art Museum (if you're into that sort of thing).
South Beach bars are entirely too touristy to recommend, but Espanola Way is slightly less so. Some restaurants have live music, and all of them have cozy, romantic interiors. If it's not in your budget, the pizza in Miami is sold by the slice and suprisingly good. Grab a drink (or 6-pack) at one of the many convenience stores and enjoy a picnic on the beach. Listen to the soundtrack of the waves crashing, the lights of the distant boats winking, and the cool sand between your toes on a perfect last night in Miami.