Alhambra tickets

That's a long side-trip from Malaga, and the Alhambra is a very large complex, not just one building. It takes multiple hours to see the Alhambra, and some people spend well over half a day there.

You'll need for the bus schedule to align with your docking schedule, etc., to make this day trip viable. Buses are somewhat more frequent than trains, and the train tickets could be quite expensive if bought late in the game. I don't know how likely it is that train tickets or bus tickets would sell out ahead of time.

Both the bus station and the train station are outside the center of town, requiring some form of transportation. Although there are trams and buses, your schedule will probably be so tight that you'll want to take a taxi straight to the Alhambra.

This is the official ticket site for the Alhambra

https://tickets.alhambra-patronato.es/en/

Tickets do sell out very early. The tickets you want are the "Alhambra General" tickets. The ticket time is for entry to the Nasrid Palaces, the most beautiful part of the complex. You're free to see other parts of the Alhambra before of after the Nasrid Palaces. If Alhambra General tickets are sold out, look at the "Dobla de Oro" tickets. They're only a bit more expensive. They include some additional, more minor sights--which you may or may not have time for, but this is a far less expensive option than a commercial tour.

If even Dobla de Oro tickets are sold out, the next-best option will probably be the Granada Card, which add a s few more sights (which you definitely won't have time for) and costs a bit more money. It will probably still be cheaper than a tour, but you can check on that. Just be sure the Granada Card allows you to obtain admission to the Alhambra on the day you need.

Malaga is quite a pretty city itself. You could spend a very nice day there without spending a lot of precious port time sitting on buses or trains.

Can you give us a little more information about the excursion you have already booked? Are they driving you by or are they letting you off for a couple of hours and expect you to buy your own tickets? Tickets are timed, so you would need to know what time you are getting there to book your entry. I booked mine last week for early May using their official website: Alhambra-patronato.es

The OP stated he has booked an excursion. My understanding is that this means that there will be a bus that takes them to the Alhambra. Public transport is not a concern.

Only the admittance to the Nasrid Palaces is on timed entry. The rest of the complex can be done at your own timing. The official ticket site is listed above. I'm sure your excursion has given information about the expected arrival and departure times at the Alhambra. Book the Nasrid Palaces portion for about 45 minutes after expected arrival. Lunch options can be a bit limited, but there is a nice cafe/restaurant in the lower level of the Parador de Granada which is part of the Alhambra Complex.

The excursion is basically the bus ride from the port to the complex. We're on our own from there.

The bus leaves the port a 8am, so I guess arrival will be around 10ish. Nasrid Palace times are 8:30 and then 1pm. I am not sure how that will work because we have to get back to port by 4, which means leaving the complex around 2ish. If I set the time for 8:30, but arrive at 10, can we still visit the palace?

Thank you all for the info.

They are somewhat strict on the timings. I would get the 1pm ticket, spend the rest of the day going around the rest of the complex, and then see as much of the Nasrid Palaces as I could in the amount of time available.