Ayup Tim
If you fancy getting some family biking in, you can ride the Coast to Coast route between Portreath and Bissoe. If you're not bringing bikes then you can hire some from Bissoe.
Otherwise, pretty much as everyone has suggested. Go for a drive along the coast between St Ives and Lands End – proper remote feeling you won't get anywhere else in Cornwall (except Bodmin Moor!). There's Paradise Park in Hayle which has all types of exotic birds (feathered variety)- not been tehre for years admitedly but used to like it when I was a kid. Talking of Hayle – the Maize Maze looked pretty barren when I drove past last week so don't bother with that. Praa Sands is nice as is Gwithian/Godrevey for beach action.
Scillies is good for a day trip, but it's a flat bottomed ship so if the sea is choppy, the 2hr or so trip over is not pleasant! You can fly over but it's pretty expensive.