HERE'S an excellent idea for a Christmas present. Lettie Farr, wife of South African Consul-General Michael Farr, has prepared - with the help of chefs from the Royal Pacific - 600 Christmas cakes for sale at $150. And here's what so special. Eighty-two of the cakes have a tiny Krugerrand coin of pure gold (one-tenth of an ounce, worth $320 at current prices) inserted in with the tasty ingredients. Proceeds from their sales will go to the work of Save the Children in Southern Africa. For more information contact Bonnie Woo (tel: 577 3279) or Carol Wright (tel: 835 2071 or fax: 572 3049).