If you want a fun MTB ride then the route I would recommend is:
Start from the High Moorland visitors car park at Princetown – SX590734 or PL20 6QF. Go back to the mini roundabout & straight across between the two pubs. The trail takes you uphill through 2 x gates & onto the moor along a very nice & well defined bridleway. Go past South Hessary Tor on your left & just follow the trail which has a number of granite water bars to roll/crash/jump over.
After about a mile at the stone marker in the middle of the trail turn right & follow the trail down the hill, it’s rocky & it has a few water crossings. Keep bearing left after going past the trees when you get to a ‘Y’ junction & take another left a bit further down, this will take you to Nosworthy Bridge.
At Nosworthy bridge go left & follow the road, after about half a mile at the T junction turn left into Sheepstor village. Follow the road until you go across the cattle grid turn left towards Nattor. The tarmac road then goes into a small car park & onto loose gravel past the 'Scout hut' on your right keep going up the trail to Eylesbarrow mine ( nice long climbing bit) then follow the trail over the granite water bars & keep going all the way back to Princetown. Should be about 2 hours depending on your fitness.
Can I recommend that you buy the OS map OL28 which covers Dartmoor, that way if you get lost you should be able to navigate your way to safety.
HTH
