Welcome to cruise-chat! The Miami port parking is so convenient and reasonable, that's your best bet. Unless you have a pre-cruise hotel stay that offers free parking.
As for location of your cabin, generally the rear of the ship is better than all of the way forward. You will be fine on deck 7 rear on the Navigator.
For doing your own independent shore excursions. Check out our sister site: http://www.caribbeanportreviews.com/