I LOVE my hometown, Miami...but why Miami? You're talking about 8-10 hours in the car -- for a relaxing day trip!
For a relaxing day at the beach, I would go to Cocoa Beach, which is only a little over an hour away from WDW. Maybe throw in Kennedy Space Center, which is in the same area.
Or, if you want a more tranquil beach, drive the other way to somewhere on the Gulf Coast (St. Pete Beach, Clearwater, etc).
Miami is an amazing city -- but NOT for a couple of hours at the beach and a little shopping. Come for 3-4 days and actually enjoy Miami!
Have to agree with you on all points here.
Miami is a great city to stay in and enjoy.
The traffic is crazy, so once in the area we like to find a place to base our stay and then travel out at off peak times and try to avoid the freeways. We find North Miami Beach/Sunny Isles Beach/Hollywood to be more reasonably priced and within driving distance of area attractions and amenities.
You say that you enjoy driving, I do too, but Miami is my least favorite city to drive in, and in to.
Approaching the city traffic gets more and more congested and getting off the turnpike or Interstate to wind your way down A1A is annoyingly slow with traffic lights every couple of blocks (all of which seem to be on red for any trip that we have taken down this route).
On I-95, the posted 70-75 mph speed limit seems to be more of a "suggestion" than the actual speed of travel with 4,5,6 lanes of traffic merging in and out.
Definitely not my idea of a "relaxing" drive.
And again I agree with JimMIA a longer stay would be a great idea but if not practical, a trip to Cocoa Beach (a nice quiet beach town) is a lot more relaxing and a pretty nice drive.
You could extend your day by driving North up A1A along the coast to Daytona Beach. A really nice drive also and Daytona is a pretty upbeat city with a great beach, pier and Boardwalk area. Lots of shops and places to eat and explore. You can even drive and park on the beach (tides permitting and there is a fee)
If you still wanted to explore more, you could continue North on A1A all the way up to St Augustine, a great touristy destination in itself. Explore the old town, the fort, the beaches, shopping etc.
Cocoa is a little quiet for our liking, but we frequently do Daytona Beach when we want a day off from the parks.
The drive up from Daytona to St Augustine along the coast is stunning and definitely worthy of a convertible.