SEATTLE INTERNATIONAL HOSTEL OPENS TODAY. The historic building that once served as the Ellis Island of the Pacific Northwest, welcoming some 900,000 immigrants annually, will make available 125 beds to members of the American Youth Hostels. Located at 84 Union St., the hostel is within easy walking distance of the Seattle Aquarium, Waterfront Park, Pioneer Square, and the ferry terminals. For reservations, call Louise Kipping at (206) 622-5443. THE FINAL 7-MILE PIECE of the 469-mile Blue Ridge Parkway, closing the gap between the towns of Blowing Rock and Linville, N.C., opens on Sept. 11. The new Linn Cove Viaduct, a 7,500-ton bridge curving around a mountain, is said to provide one of the most spectacular views along the 51-year-old highway. FOR THE 15TH YEAR, Louisville, Ky., is holding a free Bluegrass Music Festival, this time on Sept. 12-13. Among featured performers this year will be Bill Monroe and the Bluegrass Boys and New Grass Revival. One new feature is workshops conducted by performers. Kentucky theme foods will be offered and artisans will demonstrate and sell their crafts. For more information call (502) 583-1671 or write Louisville Central Area, 555 Brown & Williamson Tower, Louisville, KY 40202. NASHVILLE TERMINAL IS READY. All flights to and from Nashville, Tenn., will use the new Nashville Metropolitan Airport Terminal as of Sept. 14. It is situated across the runway from the existing terminal. Under construction since 1984, the terminal has 46 air carrier gates, 9 luggage carousels, 21 food and beverage areas, 22 restrooms, and 5,100 parking spaces.