Day Trip in Amalfi

Amalfi to Ravello is 2.5 miles. In July, temps near or in the 90s would not be 'news'. I would not choose to do that much walking in that heat.

What shore excursions does the cruise offer? Use that as a guide as what might be manageable to do within the available time.

Ferries could be cancelled if there is some unusual ( e.g., windy) condition

https://search.ricksteves.com/?button=&date_range=2y&filter=Travel+Forum&query=Amalfi&utf8=%E2%9C%93

Hopefully there is some helpful info in the above Search.

The congestion in the AC is legendary from about April thru October. Is there a reason you don't just want to explore Salerno?
The ferries can be canceled if a storm comes in. Forget the SIKA bus. Taxis are driving on the same congested roads as everyone else, so could be an issue if you have a deadline. How many hours do you have total?
What is important to you to experience?
The AC is beautiful, but a victim of its Instagram success. However, knowing the realities on the ground will help you have a memorable experience. Safe travels!

It will be HOT w/lots of steep walks and steps. Traffic can be bad and be at a complete stop sometimes.

I'd probably do a cruise excursion to be sure you are back in time and don't miss the boat. Or just explore Salerno on your own for the day.

Welcome to the forum!

Consider taking the train from Salerno to Paestum
Easy to do and should allow plenty of time at Paestum- have lunch there at one of the cafes in the shade with view of the temples. The museum is worth a visit and would at least be indoors

Taking the train means no reliance on taxis, buses or ferries

With 6 of you you will need 2 taxis

Look, you did not say this, but it's not true that only losers pay for the cruise's own expeditions. When local conditions are congested, it's worth a lot of money to know that the ship will wait if you are late to return. How important is it to see an Amalfi Coast town to you? I would say that Positano is much more "iconic" than Amalfi (town), but Positano was a bit of a disappointment to me (independent travel, from Sorrento.) I like the UNESCO WHS site Paestum, unless the ship will also be taking you to Pompeii. Pompeii is also accessible by train from Salerno, but needs two cab rides in MODERN Pompeii town, to and from the long-distance (NOT Circumvesuviana) train station.

I would definitely rank Pompeii ahead of Positano. Opinion.

It's hard to give you suggestions not knowing at what time you can get the day going. But this looks difficult. How early can you get on the ferry from Salerno to Amalfi?

Two years ago we stayed in Praiano and did excursions from there, including a ride out to Capri. Positano is very busy but it is a cute town and plenty of sites, shopping and good restaurants to keep someone entertained for a day.
There is a lot of walking to do and even those in excellent shape struggle with the climb on average streets. Enjoy!

Salerno's pretty flat, as I recall. For a mid-summer trip, I'd strongly consider seeing the Amalfi Coast from the water, with a back-up plan of exploring Salerno and maybe Paestum.