Santorini

We have been to Santorini several times. Your options are to tender to the main port below Fira and explore the capital of Fira. One thing you might want to consider is how many cruise ships are in on your date. If you get off your ship and DIY around the capital of Fira you need to keep in mind that you have to go up and down by cable car. You might have a 2 hour wait to get back to your ship.
If you take a ship excursion they tender to the ferry port and take you up by bus. The ship will wait for ship excursion people who are late. They won't wait for DIY people stuck waiting for the cable car.
This is a stop where a ships tour may be a good option.

I live on the island and I can tell you there are many things to do in one day, even though you would definitely love it even more if you stayed for at least 3-4 days!

You could start your day early by heading to Oia, a picturesque village on the northern tip of the island. While you're there you can also visit the famous Blue Domes of Oia, which are an iconic spot for taking photos!

In the afternoon you could head to the Black Beach in Perissa, which is about 30-40 minutes away from Oia.

You could end your day by watching the sunset from the village of Fira, the island's capital where there are also great places to eat.

If you need to can't you walk down instead of taking the cable car?

Yes you can walk down. There are stairs. As I recall 900 steps on marble covered by Donkey poo plus the donkey traffic. It can be a bit slippery.

600 steps. It just feels like 900. Please don't be tempted to ride the poor abused donkeys!

I will also be on a cruise in late September. What’s the best way to get from Fira to Ios. I heard it was about 5 miles. Thanks

What's the best way to get from Fira to Ios. I heard it was about 5
miles.

About 40 minutes to 1 hour by ferry. But once in Ios you have to find another one to return to Santorini. Otherwise the only plan B is to swim.

We hired a driver/guide who met us at the top of the cable car at Fira. We first drove to Ola (driver said- we do this first to avoid the crowds,) then to Akrotiri, (which we told her we really wanted to see) then to an overlook to see Red beach, then to Galleni Tavern for lunch on the beach on the East end, before driving around the island. We could have stopped at the winery, but we declined. Then she dropped us off at the south end of Fira and we walked the pedestrian walkway via the shops back to the cable car down to the dock.
We hired the driver because Oia and Akrotiri are on the opposite ends of the island and taking the bus would have been time consuming and a hassle.
If you are only going to see Oia, then taking the bus from Fira and back is possible.

@jpc I think you mean Oia (also spelled Ia) which is a town on Santorini and not IOS, which is a separate island nearby.