I really enjoyed a loop I did from Penzance round through St Ives (Obviously, you would start at St Ives):
Penzance, Newlyn, Mousehole, Raginnis, Bossava, onto B3315 at Merry Maidens, Treen, Trethewey, join the A30 at Sennen (it wasn't too busy when I was there), (worth dropping down to the lovely beach at Sennen Cove for an ice cream and a paddle, but it is a steep climb back up!), B3306 through St Just and follow all the way to St Ives (the area round Zennor is particularly lovely), B3311 to Penzance. There are lots of variations - just check the OS map. Stay off the A-roads marked in green!