Always a must return when I'm in the States, Miami is the perfect blend America, Latin culture and the beauty of the Caribbean.
For quick visits, I like to stay on South Beach, right in the heart of the action. I think some mistake South Beach for a party crazy part of Miami - it is definitely an experience, full of all sorts of nightlight and interesting characters, but it really is a great base to explore not only the South Beach area - shops, architecture and restaurants, but also greater Miami, and Florida Keys. South Beach for me is electric and eccentric.
This time I stayed at The Betsy Hotel, and absolutely stunning boutique hotel right on Ocean Drive. An oasis from what I affectionately refers to as Miami madness. Great rooms, lovely service and access to a private beach right on your front doorstep, which I definitely took full advantage of.
If you get the chance, I'd also highly recommend hiring a car and exploring surrounding Miami, including Key Biscayne, Bal Harbour (all your luxury shopping needs covered), and if you have a little more time, do yourselves a favour and make the 2.5/3 hour drive down to Key West, stopping at the many lovely and often secluded swim spots along the way.
Crystal blue waters, endless supply of palm trees and coconuts - a tropical wonderland.
In summary, when in Miami you MUST spend as much time sea-side as possible, water-sports mandatory, try some diverse and delicious and of course indulge in a little shopping.