Cruise port in Malaga.

I take cruises all the time, and the FIRST thing I do is review all the shore excursions offered by my ship, and see what interests me, and if I believe the cost is fair.
The next thing I do is read up on the port, and consult other sources for additional ideas.
No one can tell you what to do, as we do not know your interests or your budget.

If this is your first time in Spain, and you do not wish to walk around in Málaga, I would definitely splurge and take a tour to Granada and visit the Alhambra and the Generalife, gorgeous jewels in the history of the country!

Resources:

https://www.viator.com/Malaga-tours/Port-Transfers/d956-g24-c4032

https://www.getyourguide.com/malaga-l402/shore-excursions-tc156/

https://www.tripadvisor.com/Attractions-g187438-Activities-c42-t234-Malaga_Costa_del_Sol_Province_of_Malaga_Andalucia.html

If just in port for the day, other than Malaga, then Grenada is a common excursion, it does take some time to get there, seeing the Alhambra requires some good timing (you also need timed tickets, in advance), and it maybe is worth paying the cruise line for an actual excursion that includes the Alhambra (as opposed to a bus ride that gets you to Grenada) or working with a tour company in Malaga. It really depends if you only have the day, or the boat is spending the night in port.

The other common option is Ronda and the White Towns, again, best as an excursion or organized tour, maybe a rental car, public transport is available, but frequency and timing may be an issue.

Otherwise, there are a number of sights in Malaga, the town center is a large pedestrian area, a market, good restaurants and bars. There is also the Alcazaba, a castle, roman ruins, a Picasso museum. If the weather is good, there is the beach with restaurants and beach clubs to spend the day.

Depending on how much time you have and your interests— our adult kids (37/40) had a great day trip to Caminito del rey https://www.caminitodelrey.info/en/your-visit/access-route
Not my cup of tea but a highlight for them.

We did the Alhambra with rhe cruiseline, the only time we have used the cruiseline for an excursion. They insure that the ship will wait for you. In our case the tour was late getting back. However there were other tours that were late and our tour could have made it back in time if they have not had such a long stop on the way back.

In addition to the suggestions above, I like checking out Can I Do This Port:

https://www.canidothisport.com/malaga-spain

....and also Cruise Critic:

https://boards.cruisecritic.com/forum/465-spain-and-portugal-ports/
- a good post: https://boards.cruisecritic.com/topic/3099771-malaga-on-our-own/

Malaga is lovely. Obviously, it can be a cruise stop to head to Granada for the Alhambra, but as itself, it has some wonderful (like so many Spanish cities) places to see including its own "fortress" and the Picasso museum.

Walking the El Caminito del Rey was a highlight for us…as 70 somethings! We also enjoyed wandering beautiful, laidback Malaga. The Alhambra, and Granada itself, were uncomfortably crowded. Cordoba is lovely and the Mezquita is incredible. Little Frigiliana, though a bit touristy, is gorgeous. Lots of options depending on your preferences and the time available. Happy travels!

If you wish to stay in Malaga you could go to the West side of the city to the Automobile and Fashion Museum and/or the Russian Museum. Both are close to the water and there is a nice promenade along the sea and many restaurants. The Glass and Crystal Museum is in a lovely home off of Ollerias a ways away from City Center and if you're lucky one of the owners will be your tour guide. It is a beautiful museum and interesting tour.

Susan beat me to it! I second the Auto and Fashion Museum. My husband and I both enjoyed it immensely for a couple of hours while our ship was in port.

If you’ve done the old town, El Palo is a nice change, a short bus ride east for espetos, sardines grilled on skewers right on the beach. Nerja and Frigiliana pair well for sea views plus a proper white village. The Caminito del Rey is the wow option but needs timed tickets and about an hour each way, so only if you’re in port all day. Check your all aboard time before you pick