Do.Not. Under. Any. Circumstances.Whatsoever. Approach. St.Ives.By. Car.In.The.Summer. Just.....don't. Fortunately, the little train is excellent and convenient. It kind of is the park and ride. There's a massive car park at St. Erth station, for that very purpose.
Excellent that you love walking- I highly recommend the Cornish coast path, especially around the Lands End peninsula. There's an open topped bus around the coast road (think it runs PZ- St. Ives), so you can catch the bus back. Especially beautiful bits: Nanjizal, Cot Valley, Cape Cornwall, Kenidjack Valley (or all of it, really!)
I don't understand the dislike for Penzance on here, other than that it's a real town, not a tourist one. Check out the Jubilee Pool open air lido, Morrab subtropical gardens, beautiful backstreets, and pubs and quirky shops on Chapel Street. If you have bikes you can ride all along the seafront to St. Michael's Mount in one direction, and Mousehole in the other.
Neolithic monuments inland in the Lands End Peninsula- Men an Tol, Nine Maidens, Merry Maidens, Boscawen-Un stone circles, Lanyon Quoit, Carn Euny Iron Age village (and well). Buying an OS map would be a great investment!
Find out if Lafrowda Day in St. Just is on to coincide with your visit (and if it's on this year
)- it's a great little festival.
Hayle itself is pretty up and coming, and pretty interesting- it has miles of beaches, including some great dunes in places.
Day trip to the Isles of Scilly from Penzance on the Scillonian.
Porthcurno: Minack Theatre, Porthcurno Telegraph Museum if you're that way inclined, plus a stunning beach (beware, not always safe for swimming, seek local advice).