Cornwall Tours

You didn't indicate the cruise line or other details.
https://shoreexcursions.com/st_location/europe-shore-excursions/
https://cruiseweb.com/deals/cruise-deals

The OP is on the Caribbean Princess and the port is a 'Water Shuttle' [tender] port.

You need to allow extra time at the start and end of the day, and do not know what time you will be ashore, especially as an independent traveller.
I would not expect to be ashore before 9am at the earliest, and would aim to be back at the tender dock by 4.30pm.

The ship anchors in the bay -it is a 30 minute tender ride. The port is well known in the cruise world for it's team of volunteer ambassadors who will meet you as you step ashore.

From the port cruise guide try these companies-
https://absolutours.co.uk/?fbclid=IwAR34GVEFjaR6yhUtNkLPoALa_BvYS0KM6KWbusHK8PAt35zJ8v0ldsfamu0
https://www.cornwalldiscovery.co.uk/?fbclid=IwAR2_UEhCb6Jf5TsbfdquGfnzc47k2TzGHrpM9-S4H_XXsqHtOGtrvYF88Q4%3Ffbclid%3DIwAR2_UEhCb6Jf5TsbfdquGfnzc47k2TzGHrpM9-S4H_XXsqHtOGtrvYF88Q4

https://falmouthtaxis.com/?fbclid=IwAR03uulcfyN9_3HjdfbjatJwyqUdQgdMq9aAXCHGEMl60qcauxQxP9HL5bs

'[apart from being in the port guide there is no specific recommendation here]

Personally, given the constraints I would aim to stay in Falmouth for the day- there is more than plenty to occupy the day including the waterbus or land bus to Flushing *fun trip take the waterbus over and the landbus back), the ferry to St Mawes, the gardens at Trelissick. Trebah, Glendurgan, Potager (at Constantine) and Penjerrick (the former by taxi or ferry, the others by taxi or the local OTS bus), the Maritime Museum and Pendennis Castle. The welcome Ambassadors will give you maps and all the information you need.

The city of Truro is a short train or bus ride away.

The reason for query is that I'm a little under-whelmed by Princess offerings.

Looking at the Princess excursions (leaving aside the usual extortionate price tags) they seem to me to reflect the ground which you can reasonably see in the space of a day.
It doesn't matter who you hire- you simply cannot see anything vaguely like the whole of Cornwall in the space of one day. It is far too large a county for that. You need a week to do anything like justice to the County- not a day.

In particular Polperro. Lanhydrock and Fowey and the Lizard are good excursions.

If you were thinking about St Michael's Mount you would need to check that tides actually allow access to the island that day.

By the way it is nonsense to say, as they do, that no restrooms are available at Marazion/SMM.

Hank, I stayed in Falmouth for four days and given your time frame, I doubt that you would be able to get out too far from there, even with a tour.

Here is a recent post that gives some ideas for what you can do in Falmouth. It's really a lovely town with a beautiful coastline and harbor. https://community.ricksteves.com/travel-forum/england/falmouth-uk