Where abouts are you staying? Theres a few good mtb spots but I always seem to have to drive to get to the good stuff.
Dartmoor may not be far depending where you are and Gawton and Tavi woods are on the edge of Dartmoor. If your near Bodmin there are some purpose built trails at Cardinham and Lanhydrock which are good when everywhere is soaking wet. Grogley woods are fun too, also near Bodmin.