Veronica Farm's wonderful fudge is almost worth a trip to Bryher on its own. It's made using butter, milk and clotted cream (all sourced from the farm's cows, of course). You can pick up a bag at the little honesty stall outside the farmhouse, or buy it in Bryher's shop, as well as several shops on St Mary's.
The fudge is made to a traditional family recipe (and originally cooked over the farmhouse aga).