Walk to (or from) Lelant Saltings
Leave the hostel and head to Porthminster Beach. From the far end of the beach outside the restaurant pick up the SW Coast Path towards Lelant / Hayle. Follow it through Carbis Bay, over the dunes at Porthkidney Sands (dipping down the beach if you like) and on to St Uny Church where you can walk 1km of super quiet road / path to Lelant Saltings station.
Pick up the train to come back to St Ives (£1-2).
Allow 90 minutes with good walkers and no stopping or 3 hours for slower walkers with lots of stops.