You can't really go wrong with the beaches...Mawgan Porth, Perranporth, Trevone, Harlyn, Constantine, Daymer, Rock. A beach for every day! Fly a kite, make a sand sculpture, swim in the sea, investigate the rockpools (Daymer great for this).
Cardinham Woods is near Bodmin and is great. There are a number of way marked trails. A stream for kids to play in. A fab play park. An amazing Woods Cafe which is reasonably priced. All for the cost of a car park ticket.
Cycle the Camel Trail, but this is not cheap - £15 to hire a bike for each adult approximately. There are lots of places where you can park and walk a section of it though which is much less expensive.
Crabbing from Padstow Harbour and a pasty lunch.
Waterworld at Newquay is good fun for kids and fairly reasonably priced.
There is a small cinema in Wadebridge if there is a rainy day. Also a great park and recreational space for kids by the river.
Walk the coast path for stunning views. The walk round Stepper Point is one of my most favourite ones.
Loads of geocaches to find everywhere in Cornwall.
Trelawney Garden Centre in Wadebridge is good to walk around. Lots of retail items to look at. A large play park for the kids. And a good cafe that is reasonably priced.
Kingsley Village at Fraddon has a Next and a Macdonalds.
Loads of cafes in Newquay. Wetherspoons are also in Truro and Bodmin.