As well as all the above, there’s The Track at Portreath and Woody’s near Par – both said to be good. And if you’re going to Cardinham might be worth taking in Landhydrock (just a mile up the road – just ride from one to the other or you’ll end out paying twice for parking). Lanhydrock is fairly tame (so’s Cardinham, to be fair), but Hart and Timber have a nice flow to them. They’re short, though.
Another vote for Carn Marth. I tend to park just north of Unity woods and go from there, taking in Poldice too for a fun loop. Lots of cheeky too, if you’re that way inclined.
If you’re further west the Penwith moors have some good riding, but very weather dependant. It’s a horsey boggy bog in the wet!
As above, the coast path is probably best avoided at this time of year. Not much of it is bridleway anyway, though there are a few stretches, but it’ll be heaving with people. Some of it is pretty high on exposure too – we’ve had at least one mtb fatality this year, sadly, with the guy going over the edge.