Do you mean you are spending 2 nights in Miami before the cruise, or are you referring to the day the cruise departs? The cruise port is in the middle of the bay, and is not near anything else. If you are asking about what to do during the hours before you board your ship, Miami's Bayside Marketplace is a good place to hang out for a few hours, and perhaps grab a nice lunch. You will need to board your ship at least an hour before departure, and will be able to get onboard usually by noon or 1 PM on that day.
If you truly do have a full day and night there before your cruise, Miami Beach would be interesting to walk around. Or you could take a day trip to the Everglades. Check with your TA to see what (s)he can arrange.