Because Plymouth doesn’t have many cruises, you’ll find that you are greeted off the tender by a lovely group of volunteers with maps and advice on what to do. Here’s a bit about them: https://westwardshippingnews.com/the-plymouth-cruise-ship-ambassadors/
The whole Barbican area (where you’ll go ashore*) is full of history and lovely to meander around. There’s a small Mayflower Museum as well as the memorial steps. Also loads of quirky shops and cafes.
Then I suggest going up on the Hoe, which is the area around the waterfront with a broad grassy area and views of the wonderful Plymouth Sound. Smeaton’s Tower lighthouse is open for visitors.
Or you could take a harbour boat trip - boats leave from near where your tender will dock.
Or even a normal service ferry to Cawsand, a very pretty little Cornish village with pubs and a small beach.
Also: lots of good fish & chips. Harbourside are excellent but if you do takeout, watch out for seagulls.
*I think most cruise ships anchor out in the Sound and a tender brings the passengers into the Barbican area but I believe some cruise ships come into the Millbay Docks, to the west of the Hoe. Either way, the greeters will be there. If it’s the latter, you can walk over the Hoe towards the Barbican and get wonderful waterfront views.