Enys garden is very nice, it's near Penryn normally its quiet so feels like you have the place to yourself.
There's a great cafe Cornish Barista in a disused quarry which has a climbing wall, adventure type thing there, it's called Via Ferrata I think.
Crabbing off prince of Wales pier. Beerwolf which is up an alley way off market street has a second hand bookshop and board games, pinball machines. Neighbourhood pizza on killigrew street is good!
If you/ they like afternoon tea, dolly's tea room is lovely.
Get the ferry to St Mawes, Theres a playground with a skate ramp at the back of the carpark. I like the Watchhouse for lunch. There's good rockpooling there.
Stithians lake has watersports and a cafe too.
Go to glendurgan or trebah then walk the beautiful path along the river to Helford passage, there's a pebble beach, good rockpooling too and the ferryboat Inn, love the mackerel tacos there!