Hyatt Miami at the Blue is very nice (the nicest Hyatt I've stayed at, granted, I've only stayed at maybe 10 Hyatts, I'm pretty new to Hyatt), however there is nothing to do there, its out in Doral. The property is made up of a bunch of small Key West style looking buildings around a golf course. Its west of the airport, however, and is a good 20 minutes to POM.

The Hotel Victor is on South Beach, so obviously there is a lot around there. Its probably about 15 minutes to the port as well, depending on traffic.

I used to live walking distance from the Hyatt Downtown, but on the other side of the river, in Brickell. Brickell has changed a lot, and there are some things to do there at night now. Mary Brickell Village is walkable (or take free MetroMover) where there are plenty of restaurants and a few bars. Also as far as parking, if you do have a car, you can park cheaper than the Hyatt at some of the other garages nearby, but it will still cost you.

Ranked in order of stuff to do:

1) Hotel Victor
2) Hyatt Downtown
3) Hyatt Miami at the Blue