Penzance is a strong base for day trips because you can do a surprising amount without driving. That is useful when you want to keep the day flexible or avoid parking altogether.
Build the trip around the transport you actually want to use: train for longer runs, bus for straightforward local hops and taxi for the gaps in between. St Ives, Land's End and the Mount's Bay villages each fit a slightly different rhythm.
The trick is to choose one main destination and then stop the day from becoming a transport puzzle. A good day trip should still leave time for lunch, a walk and the journey back without stress.