William K. Vanderbilt II’s 24-room, Spanish-Revival mansion, Eagle’s Nest — listed on the National Register of Historic Places — sits on 43 scenic acres overlooking Northport Bay. The Museum features Vanderbilt’s collections of marine and natural-history specimens and cultural artifacts, wild-animal habitats, an observatory and the newly renovated, state-of-the-art Charles and Helen Reichert Planetarium. The Planetarium and its William and Mollie Rogers Theater — with 60-foot dome, Konica Minolta GeminiStar III projection system, full-dome video and surround-sound — is one of the finest and most advanced in the United States. Fee. 180 Little Neck Rd. Open year-round, Tue., Sat., Sun. noon-4 p.m.. Open every day in summer except Mon. Closed Mondays (631) 854-5579.