I don't know east Cornwall much either (it's been near the top of 'THE LIST' for ever) but Trethevy quoit & the Hurlers are obvious must-sees.

Dartmoor's the only bit of Devon I know at all, but I'd recommend a separate trip for sure!!! Do see Merrivale tho, as you'll be probly driving right past.

Land's Endwise I'd certainly pretty much agree with all that Oci said!

My must-sees there would be:
Boscawen-un
Chun Quoit
Merry Maidens, Gun Rith, Tregiffian Burial Chamber and the Fiddlers,
Carn Euny
Boleigh Fogou
Carn Gluze aka Ballowall Barrow (not mentioned by Oci)
Zennor quoit
Men an Tol, Men Scryfa, Nine Maidens of Boskednan, Bossiliack Barrow and Lanyon Quoit
Tregeseal circle
Mulfra quoit
Blind Fiddler
(not been to Chysauster)

Plus if you get a chance, I'd add:
Pendeen Vau fogou
Drift stones (if you're nearby)

Have fun (you will - as yer a stonehugger, it's impossible not to!) and hope the weather sorts itself out!!

love

Moth