Those are some great ideas!
From Port Orleans you can take the boats down the Disney Springs and enjoy an afternoon of shopping and dining. I love the ride itself. Such a relaxing atmosphere and a nice way to ease into a fun vacation.
Boardwalk would be a decent second choice. Though I would probably pick a different day for Boardwalk; maybe combine it with a visit to Epcot or DHS. There are a few fun things to do, but it's not as complete of an experience as Disney Springs, and the transportation options back to your hotel are limited.
Personally this would be my favorite: You could consider doing a monorail tour of the hotels in the Magic Kingdom area. That will give you a first glimpse of the Magic Kingdom, get you into Trader Sam's, and some bar hopping around the other hotels in the area. I like watching the fireworks from the TTC dock... they look so big, bright, and beautiful from there and reflecting against the lagoon. Very romantic and magical. It will give you a different experience from when you see them up-close in the Magic Kingdom.
Also, depending on how many days you're staying, you could consider upgrading your ticket to enter the parks early. Although you'll have only a few hours, the cost of the upgrade might be less than what you'd pay for similar activities elsewhere.