Before Cape Cod was a popular vacation destination, Helen J. Quinn, heiress to the Grand Union grocery chain, purchased almost 20 acres of land by the Centerville River. Several homes for her family and even a huge kennel for her dogs were built, but the most magnificent of all was Mrs. Quinn's personal estate, at 749 S Main St., later dubbed 'Sandcastle' for its sandy English stucco exterior. Brilliantly designed for hosting family or guests, the layout is timeless, with formal spaces on the entry side of the property and sunny, glassed-in rooms the length of the water side, offering 180 degree views of Craigville River and Nantucket Sound. With nine bedrooms in the main house and a full, 1400-square feet apartment above the separate four-bay carriage house, there's space for everyone. A deep water dock connected to ''the best boathouse on the river,'' makes day trips to Nantucket possible, or store a launch to zip across the river for sunbathing on the white sands of Long Beach. Upstairs in the boat house is the ideal seaside party room. When you're not feeling like boating or beaching, spend the day around the custom gunite pool with a hot tub and garden waterfall.