I would try to spend the night in HCMC if possible. You will need your passport when checking into a hotel. If you're a foodie, then a food tour is a must. There are many different ones. Some you walk from place to place, some you take a motorbike ride. So it depends on what you're comfortable with.
As somebody who knows nothing about cruises, the issue I imagine will be that since the cruise ships do offer their own itineraries, it may be hard to get independent guides/taxis/etc. I could see the cruise company not allowing outsiders near the port. You might have to walk a few blocks out to a main street to find a taxi (who will try their best to rip you off, because they know you came right off the ship). But again, we don't know where exactly you will be.
Da Nang, I would say, either
1) Get a driver to take you to the marble mountain, and then just wonder around the town the rest of the day. or
2) Get a driver to take you to Ba Na Hills (this place is not cheap though)