A lot will depend on how much actual time you have on shore. Is it overnight, e.g.: 10:00 a.m. to 10:00 a.m. the following morning, or is it only for the day that you arrive in Greenock. If it's the latter, and you have to return to the ship the same day, how much actual time will you have on the ground? Where is your next port of call after Greenock?
You probably will not have enough time to tour the Highlands by taxi or public transportation. However, stand or sit on the port side of your ship as you sail up the Firth of Clyde. You'll see Goatfell, the highest peak on the island of Arran, the mountains of the Cowal Peninsula, and as the ship turns east in to the River Clyde, if it's a clear day, you'll see the peak of Ben Lomond way off in the distance. The high plateau on the north side of the Clyde is the Kilpatrick Hills. Technically, you have crossed the Highland fault line when you're halfway up the island of Arran, so Goatfell and the Cowal Peninsula are in the Highlands.
If you decide that you'd like to go to Edinburgh, it's about 2 1/4 hours by train, changing trains in Glasgow. Has your tour company arranged for any ground transportation while you're in Scotland, or have they left you to your own devices?
Best wishes for your travels, and Happy New Year!
Mike (Auchterless) .