Tavistock is quite some distance from any beach. The nearest would be, I guess, around the Plymouth area: Wembury, Bovisand and such like in the South Hams, or across into Cornwall at Whitsand Bay.
Brentor is a small village north of Tavistock and is worth a visit. The parish church is perched on top of the tor!
Princetown is just up the road - home to a good visitor centre, some cafes and pubs and, of course, the prison! The prison does have a museum (on opposite side of road to prison) which is open to the public.
As you go onto the moor from the Tavistock direction, you will come across a large car park. In the corner is a view finder thingy which shows that, on a clear day, you can see for miles. If you park here, you can walk up Cox Tor. Again, a magnificent view from the top.
Lydford Gorge is worth a visit, the waterfalls are very pretty. Cafes here too.
Widecombe on the Moor is home to Uncle Tom Cobley. Another village worth a look at. Hay Tor is over that side, too.
Okehampton is a market town nearby, again on the moor. I think there is a steam train ride to Exeter from here on a Sunday. Check out www.dartmoor-railway.co.uk/
Hope this helps. Have a lovely time.