Those are good links others have sent. Just FYI in case you don't know, Miami really doesn't have what most here regard as great diving. Most of the diving here is just north of Miami-Dade County in Broward County or in the Keys.
I always recommend the Keys if you have the time, and it looks like you do. The most diving is arguably in the upper Keys, so you're going to be pretty close already. The wrecks there are widely regarded as the best.
Many of us dive with Island Ventures, located at about mile marker 104 on the right (bay side) headed south on A1A. Look for Sundowners and Senior Frejoles and pull into their parking lot. I don't think it would take you more than an hour and a half, but Brian would be best to ask.
Island Ventures: Brian & Carrie Berry
MileMarker:104.0
www.islandventure.com
Email:
Dive@islandventure.com
Phone: 305-451-4957
Toll Free: 1-866-293-5006
They one of many exciting wreck dives (
http://www.islandventure.com/wreck.htm) first followed by a nice, easy reef.
Afterward, have lunch or dinner at Sundowners right next door... if you do the afternoon trip and make a reservation for a "bayside table" you'll usually have an incredible sunset view. The food is always good.
There are lots of other options, which I'm sure others will contribute. If you think you'd rather head north to Broward just post and we'll give some more suggestions.
You already probably thought about your fly-time, but if not and it concerns you there is some good shore diving in Broward less than 25'. I do a shallow reef and fly the next day often.