Newlyn and Mousehole are close enough to Penzance to work as a half-day or full-day extension, especially if you are happy to mix walking with a return bus or taxi. That makes them ideal for people who want a coastal day without driving all over the place.
Plan the route so the walk follows the harbour or coast for as long as it stays comfortable, then treat the return journey as part of the day rather than a problem to solve. That leaves more energy for the harbourside, the views and the stops in between.
The best pace is deliberately slow. These are places that reward sitting down, looking out and taking your time rather than racing through with a list in hand.